Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 2,989	$ 11,897	$ 13,589	$ 12,079	$ 12,533	$ 12,199	$ 9,405	$ 10,674	$ 40,554	$ 44,811	$ 43,818
Other comprehensive income (loss), net of tax:
Unrealized holding gain (losses) on securities arising during the period									3,165	(6,384)	(1,098)
Change in minimum pension liability									(370)	592	1,246
Change in joint beneficiary agreement liability									(110)	(343)	277
Other comprehensive income (loss)									2,685	(6,135)	425
Comprehensive income									$ 43,239	$ 38,676	$ 44,243