Comprehensive Income - AOCI (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Accumulated Comprehensive Income [Roll Forward]
Beginning balance	$ (99,307)	$ (51,357)
Other comprehensive income before reclassifications	71,302	39,189
Amounts reclassified from accumulated other comprehensive income (loss)	(3,941)	(15,486)
Other comprehensive income, net of tax	67,361	23,703
Ending balance	(31,946)	(27,654)
Unrealized Gains (Losses) on Securities Available for Sale and Transferred to Held to Maturity
Accumulated Comprehensive Income [Roll Forward]
Beginning balance	(56,326)	4,469
Other comprehensive income before reclassifications	68,264	36,588
Amounts reclassified from accumulated other comprehensive income (loss)	(4,765)	(16,766)
Other comprehensive income, net of tax	63,499	19,822
Ending balance	7,173	24,291
Accumulated Gains (Losses) on Cash Flow Hedging Instruments
Accumulated Comprehensive Income [Roll Forward]
Beginning balance	(6,378)	(7,189)
Other comprehensive income before reclassifications	3,038	2,650
Amounts reclassified from accumulated other comprehensive income (loss)	(329)	(591)
Other comprehensive income, net of tax	2,709	2,059
Ending balance	(3,669)	(5,130)
Defined Benefit Plan Adjustment
Accumulated Comprehensive Income [Roll Forward]
Beginning balance	(29,166)	(41,121)
Other comprehensive income before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	931	1,715
Other comprehensive income, net of tax	931	1,715
Ending balance	(28,235)	(39,406)
Unamortized Impairment Losses on Investment Securities Held to Maturity
Accumulated Comprehensive Income [Roll Forward]
Beginning balance	(7,437)	(7,516)
Other comprehensive income before reclassifications	0	(49)
Amounts reclassified from accumulated other comprehensive income (loss)	222	156
Other comprehensive income, net of tax	222	107
Ending balance	$ (7,215)	$ (7,409)