5. Securities Available-for-Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Net Investment Income [Line Items]
Available-for-Sale Securities, Amortized cost	$ 357,190	$ 362,724
Available-for-Sale Securities, Unrealized gains	1,955	2,098
Available-for-Sale Securities, Unrealized losses	(3,288)	(4,717)
Available-for-Sale Securities, Fair value	355,857	360,105
US Treasury Securities [Member]
Net Investment Income [Line Items]
Available-for-Sale Securities, Amortized cost	1,989	977
Available-for-Sale Securities, Unrealized gains	0	10
Available-for-Sale Securities, Unrealized losses	(14)	0
Available-for-Sale Securities, Fair value	1,975	987
Obligations of U.S. Government Agencies [Member]
Net Investment Income [Line Items]
Available-for-Sale Securities, Amortized cost	42,247	60,773
Available-for-Sale Securities, Unrealized gains	10	112
Available-for-Sale Securities, Unrealized losses	(434)	(340)
Available-for-Sale Securities, Fair value	41,823	60,545
Mortgage-backed Securities [Member]
Net Investment Income [Line Items]
Available-for-Sale Securities, Amortized cost	121,087	85,709
Available-for-Sale Securities, Unrealized gains	421	397
Available-for-Sale Securities, Unrealized losses	(1,716)	(1,822)
Available-for-Sale Securities, Fair value	119,792	84,284
Asset-backed securities
Net Investment Income [Line Items]
Available-for-Sale Securities, Amortized cost	3,734
Available-for-Sale Securities, Unrealized gains	0
Available-for-Sale Securities, Unrealized losses	(48)
Available-for-Sale Securities, Fair value	3,686
Obligations of States and Political Subdivisions [Member]
Net Investment Income [Line Items]
Available-for-Sale Securities, Amortized cost	150,724	151,988
Available-for-Sale Securities, Unrealized gains	1,325	1,458
Available-for-Sale Securities, Unrealized losses	(946)	(1,828)
Available-for-Sale Securities, Fair value	151,103	151,618
Corporate Debt [Member]
Net Investment Income [Line Items]
Available-for-Sale Securities, Amortized cost	37,409	63,277
Available-for-Sale Securities, Unrealized gains	199	121
Available-for-Sale Securities, Unrealized losses	(130)	(727)
Available-for-Sale Securities, Fair value	$ 37,478	$ 62,671

FNB BANCORP/CA/ 10-K