Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 264,670	$ 176,100	$ 159,366
Other comprehensive income (loss), before tax:
Unrealized gains (losses) arising during the period	(2,231)	3,111	2,599
Amount reclassified from accumulated other comprehensive loss	(64)	60	180
Total minimum pension liability	(2,295)	3,171	2,779
Other-than-temporary impairment loss on held-to-maturity securities:
Non-credit loss on other-than-temporary impairment		(6,543)
Amount reclassified from accumulated other comprehensive loss	333
Total non-credit loss on other-than-temporary impairment	333	(6,543)
Other comprehensive income (loss), before tax	(1,962)	(3,372)	2,779
Income tax (provision) benefit related to items of comprehensive income (loss)	630	(1,052)
Other comprehensive income (loss), net of tax	(1,332)	(4,424)	2,779
Comprehensive income	$ 263,338	$ 171,676	$ 162,145