Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 176,100	$ 159,366	$ 111,511
Other comprehensive income (loss), before tax:
Unrealized gains arising during the period	3,111	2,599	1,920
Amount reclassified from accumulated other comprehensive loss	60	180	88
Total Minimum pension liability	3,171	2,779	2,008
Other-than-temporary impairment loss on held-to-maturity securities:
Non-credit loss on other-than-temporary impairment	(6,543)
Total Non-credit loss on other-than-temporary impairment	(6,543)
Other comprehensive income (loss), before tax	(3,372)	2,779	2,008
Income tax expense related to items of comprehensive income (loss)	(1,052)		(724)
Other comprehensive income (loss), net of tax	(4,424)	2,779	1,284
Comprehensive income	$ 171,676	$ 162,145	$ 112,795