CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Net income	$ 95,916	$ 66,593	$ 53,091
Net unrealized gain (loss) on securities available-for-sale:
Net unrealized holding gain (loss) on securities arising during the year	9,143	(10,485)	(1,320)
Reclassification adjustment for gain on sales of securities included in net income	(91)	(1,332)	(5,490)
Net unrealized gain (loss) on securities available-for-sale	9,052	(11,817)	(6,810)
Net actuarial loss adjustment on pension plans and other postretirement benefits:
Net actuarial loss adjustment arising during the year	(37,467)	44,180	9,143
Reclassification adjustment for net actuarial loss included in net income	591	2,335	3,527
Net actuarial loss adjustment on pension plans and other postretirement benefits	(36,876)	46,515	12,670
Prior service cost adjustment on pension plans and other postretirement benefits:
Prior service cost adjustment arising during the year	0	0	(3,538)
Reclassification adjustment for prior service cost included in net income	123	142	98
Prior service cost adjustment on pension plans and other postretirement benefits	123	142	(3,440)
Reclassification adjustment for loss on cash flow hedge included in net income	0	0	151
Total other comprehensive (loss) income, net of tax	(27,701)	34,840	2,571
Comprehensive income	$ 68,215	$ 101,433	$ 55,662