CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net income	$ 88,075	$ 95,916	$ 66,593
Net unrealized (loss) gain on securities available-for-sale:
Net unrealized holding (loss) gain on securities arising during the year	(1,816)	9,143	(10,485)
Reclassification adjustment for gain on sales of securities included in net income	(43)	(91)	(1,332)
Net unrealized (loss) gain on securities available-for-sale	(1,859)	9,052	(11,817)
Net actuarial loss adjustment on pension plans and other postretirement benefits:
Net actuarial loss adjustment arising during the year	7,309	(37,467)	44,180
Reclassification adjustment for net actuarial loss included in net income	1,771	591	2,335
Net actuarial loss adjustment on pension plans and other postretirement benefits	9,080	(36,876)	46,515
Reclassification adjustment for prior service cost on pension plans and other postretirement benefits included in net income	113	123	142
Total other comprehensive income (loss), net of tax	7,334	(27,701)	34,840
Comprehensive income	$ 95,409	$ 68,215	$ 101,433