Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 18,906	$ 18,808	$ 69,644	$ 72,657
Net unrealized gain (loss) on securities available-for-sale:
Net unrealized gain (loss) on securities available-for-sale:	2,903	(420)	2,148	7,272
Reclassification adjustment for gain on sales of securities included in net income	0	(91)	(43)	(91)
Net unrealized gain (loss) on securities available-for-sale	2,903	(511)	2,105	7,181
Reclassification adjustment for net actuarial loss on pension plans and other postretirement benefits included in net income	442	147	1,328	443
Reclassification adjustment for prior service cost on pension plans and other postretirement benefits included in net income	29	31	85	92
Total other comprehensive income (loss), net of tax	3,374	(333)	3,518	7,716
Comprehensive income	$ 22,280	$ 18,475	$ 73,162	$ 80,373