Accumulated Other Comprehensive Loss - Changes (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pre-tax Amount
Balance	$ (225,385)	$ (143,296)	$ (159,423)	$ (145,472)
Other comprehensive income (loss)	(24,263)	31,473	(90,225)	33,649
Balance	(249,648)	(111,823)	(249,648)	(111,823)
Income Tax Benefit (Expense)
Balance	60,701	56,604	63,040	57,461
Other comprehensive income (loss)	6,537	(12,432)	24,266	(13,289)
Balance	67,238	44,172	67,238	44,172
Net of tax
Balance			2,532,551	2,476,485
Other comprehensive (loss) income	(17,726)	19,041	(65,959)	20,360
Balance	2,459,175	2,552,602	2,459,175	2,552,602
Accumulated other comprehensive loss
Net of tax
Balance	(164,684)	(86,692)	(96,383)	(88,011)
Other comprehensive (loss) income	(17,726)	19,041	(65,959)	20,360
Balance	(182,410)	(67,651)	(182,410)	(67,651)
Accumulated other comprehensive loss \| ASU 2018-02
Income Tax Benefit (Expense)
Adoption of ASU	(20,068)		(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)		(20,068)
Investment securities:
Pre-tax Amount
Unrealized net (losses) gains arising during the period	(24,688)	31,215	(91,388)	32,793
Other comprehensive income (loss)	(24,688)	31,215	(91,388)	32,793
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period	6,649	(12,330)	24,572	(12,951)
Other comprehensive income (loss)	6,649	(12,330)	24,572	(12,951)
Net of tax
Balance	(138,334)	(59,001)	(74,117)	(59,958)
Unrealized net gains (losses) arising during the period	(18,039)	18,885	(66,816)	19,842
Other comprehensive (loss) income	(18,039)	18,885	(66,816)	19,842
Balance	(156,373)	(40,116)	(156,373)	(40,116)
Investment securities: \| Adjustments for New Accounting Principle, Early Adoption \| ASU 2018-02
Income Tax Benefit (Expense)
Adoption of ASU	(15,440)		(15,440)
Cash flow derivative hedges:
Pre-tax Amount
Unrealized net (losses) gains arising during the period	425	258	1,163	856
Other comprehensive income (loss)	425	258	1,163	856
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period	(112)	(102)	(306)	(338)
Other comprehensive income (loss)	(112)	(102)	(306)	(338)
Net of tax
Balance	4,989	2,546	3,680	2,184
Unrealized net gains (losses) arising during the period	313	156	857	518
Other comprehensive (loss) income	313	156	857	518
Balance	5,302	$ 2,702	5,302	$ 2,702
Cash flow derivative hedges: \| Adjustments for New Accounting Principle, Early Adoption \| ASU 2018-02
Income Tax Benefit (Expense)
Adoption of ASU	$ 765		$ 765

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Amount before tax of increase (decrease) in accumulated equity from transactions and other events and circumstances from non-owner sources, attributable to parent. Excludes net income (loss), and accumulated changes in equity from transactions resulting from investments by owners (distributions to owners).

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