CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 71,529	$ 88,075	$ 95,916
Net unrealized (loss) gain on securities available-for-sale:
Net unrealized holding (loss) gain on securities arising during the year	(515)	(1,816)	9,143
Reclassification adjustment for gain on sales of securities included in net income	(51)	(43)	(91)
Net unrealized (loss) gain on securities available-for-sale	(566)	(1,859)	9,052
Net actuarial loss adjustment on pension plans and other postretirement benefits:
Net actuarial loss adjustment arising during the year	765	7,309	(37,467)
Reclassification adjustment for net actuarial loss included in net income	1,424	1,771	591
Net actuarial loss adjustment on pension plans and other postretirement benefits	2,189	9,080	(36,876)
Reclassification adjustment for prior service cost on pension plans and other postretirement benefits included in net income	113	113	123
Total other comprehensive income (loss), net of tax	1,736	7,334	(27,701)
Comprehensive income	$ 73,265	$ 95,409	$ 68,215