Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 18,336	$ 31,431	$ 36,901	$ 50,738
Net unrealized gain (loss) on securities available-for-sale:
Net unrealized holding gain (loss) on securities arising during the period	507	(2,748)	4,543	(755)
Reclassification adjustment for gain on sales of securities included in net income	0	(43)	(51)	(43)
Net unrealized gain (loss) on securities available-for-sale	507	(2,791)	4,492	(798)
Reclassification adjustment for net actuarial loss on pension plans and other postretirement benefits included in net income	315	417	712	886
Reclassification adjustment for prior service cost on pension plans and other postretirement benefits included in net income	29	27	57	56
Total other comprehensive income (loss), net of tax	851	(2,347)	5,261	144
Comprehensive income	$ 19,187	$ 29,084	$ 42,162	$ 50,882