Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 47,604	$ 23,605
Other comprehensive income, net of tax:
Unrealized holding gains on securities available-for-sale arising during the period	2,739	4,199
Reclassification adjustment for gains on realized income	(90)	(19)
Tax effect	(927)	(1,463)
Net unrealized gains on securities available-for-sale, net of tax	1,722	2,717
Adjustment for pension and other postretirement benefits	537	(577)
Tax effect	(188)	202
Net adjustment for pension and other postretirement benefits	349	(375)
Other comprehensive income, net of tax	2,071	2,342
Total comprehensive income, net of tax	$ 49,675	$ 25,947