Comprehensive Income - AOCI (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accumulated Comprehensive Income [Roll Forward]
Balance, beginning of period	$ 12,750,707	$ 12,624,709
Other comprehensive income, net of tax	17,471	67,391
Balance, end of period	12,885,499	12,723,177
Unrealized Gains (Losses) on Securities Available for Sale and Transferred to Held to Maturity
Accumulated Comprehensive Income [Roll Forward]
Balance, beginning of period	(119,562)	(56,326)
Other comprehensive income (loss) before reclassifications	26,933	68,264
Amounts reclassified from accumulated other comprehensive income (loss)	666	(4,765)
Other comprehensive income, net of tax	27,599	63,499
Balance, end of period	(91,963)	7,173
Accumulated Gains (Losses) on Cash Flow Hedging Instruments
Accumulated Comprehensive Income [Roll Forward]
Balance, beginning of period	(10,080)	(6,407)
Other comprehensive income (loss) before reclassifications	(6,285)	3,068
Amounts reclassified from accumulated other comprehensive income (loss)	(3,581)	(329)
Other comprehensive income, net of tax	(9,866)	2,739
Balance, end of period	(19,946)	(3,668)
Defined Benefit Plan Adjustment
Accumulated Comprehensive Income [Roll Forward]
Balance, beginning of period	(32,028)	(29,166)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	(485)	931
Other comprehensive income, net of tax	(485)	931
Balance, end of period	(32,513)	(28,235)
Unamortized Impairment Losses on Investment Securities Held to Maturity
Accumulated Comprehensive Income [Roll Forward]
Balance, beginning of period	(6,582)	(7,437)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	223	222
Other comprehensive income, net of tax	223	222
Balance, end of period	(6,359)	(7,215)
Accumulated Other Comprehensive Loss
Accumulated Comprehensive Income [Roll Forward]
Balance, beginning of period	(168,252)	(99,336)
Other comprehensive income (loss) before reclassifications	20,648	71,332
Amounts reclassified from accumulated other comprehensive income (loss)	(3,177)	(3,941)
Other comprehensive income, net of tax	17,471	67,391
Balance, end of period	$ (150,781)	$ (31,945)