Comprehensive Income - Reclassifications out of AOCI (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest on debt securities held to maturity		$ 12,426	$ 6,639
Interest and fees on loans		663,935	574,712
Interest and fees on FHLB advances		(24,756)	(19,068)
Net income before income tax expense		260,488	166,669
Income tax (expense) benefit		(51,798)	(45,846)
Net income		208,690	120,823
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Unrealized Gains (Losses) on Debt Securities Available for Sale and Transferred to Held to Maturity
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest on debt securities held to maturity	[1]	(2,639)	(1,058)
Income tax (expense) benefit	[1]	620	392
Net income	[1]	(2,019)	(666)
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Accumulated Gains (Losses) on Cash Flow Hedging Instruments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest and fees on loans	[1]	(8,890)	6,374
Interest and fees on FHLB advances	[1]	(495)	(680)
Net income before income tax expense	[1]	(9,385)	5,694
Income tax (expense) benefit	[1]	2,215	(2,113)
Net income	[1]	(7,170)	3,581
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Defined Benefit Plan Adjustment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net periodic expense	[1],[2]	4,425	773
Income tax (expense) benefit	[1]	(1,046)	(288)
Net income	[1]	3,379	485
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Unamortized Impairment Losses on Debt Securities Held to Maturity
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest on debt securities held to maturity	[1]	(171)	(354)
Income tax (expense) benefit	[1]	41	131
Net income	[1]	$ (130)	$ (223)

[1] Amounts in parentheses indicate debits to the Unaudited Condensed Consolidated Statements of Income.

[2] These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see Note 18, Benefit Plans, in the Notes to the December 31, 2017, Consolidated Financial Statements for additional details).