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	$ 83,722	$ 73,631	$ 70,534
Unrealized gains and losses on securities available for sale:
Net unrealized (losses) gains arising during the period	(3,401)	10,692	(19,428)
Reclassification adjustment for gains included in net income	(391)	(150)	(589)
Total	(3,792)	10,542	(20,017)
Other-than-temporary impairment on debt securities available for sale:
Other-than-temporary impairment losses on securities	0	0	0
Reclassification adjustment for impairment losses included in net income	0	0	257
Total	0	0	257
Unrealized losses on derivatives	(73)	0	0
Amortization related to post-retirement obligations	1,290	(5,662)	7,193
Total other comprehensive (loss) income	(2,575)	4,880	(12,567)
Total comprehensive income	$ 81,147	$ 78,511	$ 57,967