Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 18,565	$ 19,307
Net unrealized gain on securities available-for-sale:
Net unrealized holding gain on securities arising during the period	4,036	1,993
Reclassification adjustment for gain on sales of securities included in net income	(51)	0
Net unrealized gain on securities available-for-sale	3,985	1,993
Reclassification adjustment for net actuarial loss on pension plans and other postretirement benefits included in net income	397	469
Reclassification adjustment for prior service cost on pension plans and other postretirement benefits included in net income	28	29
Total other comprehensive income, net of tax	4,410	2,491
Comprehensive income	$ 22,975	$ 21,798