Accumulated Other Comprehensive Loss - Changes (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pre-tax Amount
Balance	$ (159,423)	$ (145,472)
Other comprehensive income (loss)	(65,962)	2,176
Balance	(225,385)	(143,296)
Income Tax Benefit (Expense)
Balance	63,040	57,461
Other comprehensive income (loss)	17,729	(857)
Balance	60,701	56,604
Net of tax
Balance	2,532,551	2,476,485
Other comprehensive (loss) income	(48,233)	1,319
Balance	2,520,862	2,505,994
Accumulated other comprehensive loss
Net of tax
Balance	(96,383)	(88,011)
Other comprehensive (loss) income	(48,233)	1,319
Balance	(164,684)	(86,692)
Accumulated other comprehensive loss | ASU 2018-02
Income Tax Benefit (Expense)
Adoption of ASU	(20,068)
Accumulated other comprehensive loss | Adjustments for New Accounting Principle, Early Adoption | ASU 2018-02
Income Tax Benefit (Expense)
Adoption of ASU	(20,068)
Investment securities:
Pre-tax Amount
Unrealized net (losses) gains arising during the period	(66,700)	1,578
Other comprehensive income (loss)	(66,700)	1,578
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period	17,923	(621)
Other comprehensive income (loss)	17,923	(621)
Net of tax
Balance	(74,117)	(59,958)
Unrealized net gains (losses) arising during the period	(48,777)	957
Other comprehensive (loss) income	(48,777)	957
Balance	(138,334)	(59,001)
Investment securities: | Adjustments for New Accounting Principle, Early Adoption | ASU 2018-02
Income Tax Benefit (Expense)
Adoption of ASU	(15,440)
Cash flow derivative hedges:
Pre-tax Amount
Unrealized net (losses) gains arising during the period	738	598
Other comprehensive income (loss)	738	598
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period	(194)	(236)
Other comprehensive income (loss)	(194)	(236)
Net of tax
Balance	3,680	2,184
Unrealized net gains (losses) arising during the period	544	362
Other comprehensive (loss) income	544	362
Balance	4,989	$ 2,546
Cash flow derivative hedges: | Adjustments for New Accounting Principle, Early Adoption | ASU 2018-02
Income Tax Benefit (Expense)
Adoption of ASU	$ 765