Comprehensive Income - AOCI (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Accumulated Comprehensive Income [Roll Forward]
Balance, beginning of period	$ 13,013,310	$ 12,750,707
Other comprehensive (loss) income, net of tax	(74,013)	17,471
Balance, end of period	13,144,139	12,885,499
Unrealized Gains (Losses) on Debt Securities Available for Sale and Transferred to Held to Maturity
Accumulated Comprehensive Income [Roll Forward]
Balance, beginning of period	(132,821)	(119,562)
Other comprehensive income (loss) before reclassifications	(72,346)	26,933
Cumulative effect from adoption of ASU 2016-01			$ (13)
Amounts reclassified from accumulated other comprehensive income (loss)	2,019	666
Other comprehensive (loss) income, net of tax		27,599
Net current period other comprehensive loss	(70,340)
Balance, end of period	(203,161)	(91,963)
Accumulated Gains (Losses) on Cash Flow Hedging Instruments
Accumulated Comprehensive Income [Roll Forward]
Balance, beginning of period	(24,765)	(10,080)
Other comprehensive income (loss) before reclassifications	(7,407)	(6,285)
Amounts reclassified from accumulated other comprehensive income (loss)	7,170	(3,581)
Other comprehensive (loss) income, net of tax	(237)	(9,866)
Net current period other comprehensive loss	(237)
Balance, end of period	(25,002)	(19,946)
Defined Benefit Plan Adjustment
Accumulated Comprehensive Income [Roll Forward]
Balance, beginning of period	(34,228)	(32,028)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	(3,379)	(485)
Other comprehensive (loss) income, net of tax	(3,379)	(485)
Net current period other comprehensive loss	(3,379)
Balance, end of period	(37,607)	(32,513)
Unamortized Impairment Losses on Debt Securities Held to Maturity
Accumulated Comprehensive Income [Roll Forward]
Balance, beginning of period	(5,591)	(6,582)
Other comprehensive income (loss) before reclassifications	(200)	0
Amounts reclassified from accumulated other comprehensive income (loss)	130	223
Other comprehensive (loss) income, net of tax		223
Net current period other comprehensive loss	(70)
Balance, end of period	(5,661)	(6,359)
Accumulated Other Comprehensive Loss
Accumulated Comprehensive Income [Roll Forward]
Balance, beginning of period	(197,405)	(168,252)
Other comprehensive income (loss) before reclassifications	(79,953)	20,648
Cumulative effect from adoption of ASU 2016-01			$ (13)
Amounts reclassified from accumulated other comprehensive income (loss)	5,940	(3,177)
Other comprehensive (loss) income, net of tax	(74,013)	17,471
Net current period other comprehensive loss	(74,026)
Balance, end of period	$ (271,431)	$ (150,781)