Comprehensive Income - Changes in Components of OCI (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Other Comprehensive Income (Loss), Pretax [Abstract]
Other comprehensive (loss) income	$ (50,947)	$ 68,815	$ 481,277	$ 377,877
Other Comprehensive Income (Loss), Tax Expense/(Benefit) [Abstract]
Other comprehensive (loss) income	(12,156)	16,314	114,831	89,584
Other Comprehensive Income (Loss), After-Tax [Abstract]
Change in unamortized net holding gains on debt securities held to maturity	(1,644)	(2,223)	(4,924)	(5,905)
Change in unamortized non-credit related impairment on debt securities held to maturity	(102)	(175)	(341)	(675)
Change in defined benefit plans	0	0	(1,754)	(3,119)
Other comprehensive (loss) income, net of tax	(38,791)	52,501	366,446	288,293
Unrealized Gains (Losses) on Debt Securities Available for Sale and Transferred to Held to Maturity
Other Comprehensive Income (Loss), After-Tax [Abstract]
Unrealized holding (losses) gains arising during period from debt securities available for sale			125,103	169,868
Less: reclassification adjustment for net gains on sale of debt securities in net income			12,296	16,952
Other comprehensive (loss) income, net of tax			112,807	152,916
Accumulated Gains (Losses) on Cash Flow Hedging Instruments
Other Comprehensive Income (Loss), Pretax [Abstract]
Other comprehensive (loss) income	(44,951)	44,121	330,370	172,571
Other Comprehensive Income (Loss), Tax Expense/(Benefit) [Abstract]
Other comprehensive (loss) income	(10,725)	10,459	78,826	40,906
Other Comprehensive Income (Loss), After-Tax [Abstract]
Unrealized holding (losses) gains arising during period from debt securities available for sale			313,311	129,110
Less: reclassification adjustment for net gains on sale of debt securities in net income			61,767	(2,555)
Other comprehensive (loss) income, net of tax	(34,226)	33,662	251,544	131,665
Defined Benefit Plan Adjustment
Other Comprehensive Income (Loss), Pretax [Abstract]
Change in defined benefit plans	0	0	2,301	4,089
Other Comprehensive Income (Loss), Tax Expense/(Benefit) [Abstract]
Change in defined benefit plans	0	0	547	970
Other Comprehensive Income (Loss), After-Tax [Abstract]
Unrealized holding (losses) gains arising during period from debt securities available for sale			0	0
Less: reclassification adjustment for net gains on sale of debt securities in net income			(1,754)	(3,119)
Change in defined benefit plans	0	0	1,754	3,119
Other comprehensive (loss) income, net of tax			1,754	3,119
Available-for-sale Securities | Unrealized Gains (Losses) on Debt Securities Available for Sale and Transferred to Held to Maturity
Other Comprehensive Income (Loss), Pretax [Abstract]
Unrealized holding (losses) gains arising during period from debt securities available for sale	(8,290)	42,553	164,304	222,660
Less: reclassification adjustment for net gains on sale of debt securities in net income	0	21,003	22,616	29,961
Other comprehensive (loss) income	(8,290)	21,550	141,688	192,699
Other Comprehensive Income (Loss), Tax Expense/(Benefit) [Abstract]
Unrealized holding (losses) gains arising during period from debt securities available for sale	(1,979)	10,089	39,201	52,792
Less: reclassification adjustment for net gains on sale of debt securities in net income	0	4,980	5,396	7,104
Other comprehensive (loss) income	(1,979)	5,109	33,805	45,688
Other Comprehensive Income (Loss), After-Tax [Abstract]
Unrealized holding (losses) gains arising during period from debt securities available for sale	(6,311)	32,464	125,103	169,868
Less: reclassification adjustment for net gains on sale of debt securities in net income	0	16,023	17,220	22,857
Other comprehensive (loss) income, net of tax	(6,311)	16,441	107,883	147,011
Held-to-maturity Securities | Change in unamortized net holding gains on debt securities held to maturity
Other Comprehensive Income (Loss), Pretax [Abstract]
Change in unamortized net holding gains on debt securities held to maturity	2,160	2,915	6,469	7,741
Other Comprehensive Income (Loss), Tax Expense/(Benefit) [Abstract]
Change in unamortized net holding gains on debt securities held to maturity	516	692	1,545	1,836
Other Comprehensive Income (Loss), After-Tax [Abstract]
Change in unamortized net holding gains on debt securities held to maturity	1,644	2,223	4,924	5,905
Held-to-maturity Securities | Change in unamortized non-credit related impairment on debt securities held to maturity
Other Comprehensive Income (Loss), Pretax [Abstract]
Change in unamortized non-credit related impairment on debt securities held to maturity	134	229	449	885
Other Comprehensive Income (Loss), Tax Expense/(Benefit) [Abstract]
Change in unamortized non-credit related impairment on debt securities held to maturity	32	54	108	210
Other Comprehensive Income (Loss), After-Tax [Abstract]
Change in unamortized non-credit related impairment on debt securities held to maturity	102	175	341	675
Held-to-maturity Securities | Unrealized Gains (Losses) on Debt Securities Held to Maturity
Other Comprehensive Income (Loss), Pretax [Abstract]
Less: reclassification adjustment for net gains on sale of debt securities in net income	0	0	0	108
Other comprehensive (loss) income	2,294	3,144	6,918	8,518
Other Comprehensive Income (Loss), Tax Expense/(Benefit) [Abstract]
Less: reclassification adjustment for net gains on sale of debt securities in net income	0	0	0	26
Other comprehensive (loss) income	548	746	1,653	2,020
Other Comprehensive Income (Loss), After-Tax [Abstract]
Less: reclassification adjustment for net gains on sale of debt securities in net income	0	0	0	82
Other comprehensive (loss) income, net of tax	$ 1,746	$ 2,398	$ 5,265	$ 6,498