Comprehensive Income - Changes in Components of OCI (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Other Comprehensive Income (Loss), Pretax [Abstract]
Other comprehensive income	$ 54,932	$ 211,879	$ 532,224	$ 309,062
Other Comprehensive Income (Loss), Tax Expense/(Benefit) [Abstract]
Other comprehensive income	13,106	50,236	126,987	73,270
Other Comprehensive Income (Loss), After-Tax [Abstract]
Change in unamortized net holding losses on debt securities held to maturity	(1,705)	(1,939)	(3,280)	(3,682)
Change in unamortized non-credit related impairment on debt securities held to maturity	(121)	(132)	(239)	(500)
Change in defined benefit plans	0	0	(1,754)	(3,119)
Other comprehensive income, net of tax	41,826	161,643	405,237	235,792
Unrealized Gains (Losses) on Debt Securities Available for Sale and Transferred to Held to Maturity
Other Comprehensive Income (Loss), After-Tax [Abstract]
Unrealized holding gains arising during period from debt securities available for sale			131,414	137,404
Less: reclassification adjustment for net gains on sale of debt securities in net income			13,940	3,152
Other comprehensive income, net of tax			117,474	134,252
Accumulated Gains (Losses) on Cash Flow Hedging Instruments
Other Comprehensive Income (Loss), Pretax [Abstract]
Other comprehensive income	14,358	96,932	375,321	128,450
Other Comprehensive Income (Loss), Tax Expense/(Benefit) [Abstract]
Other comprehensive income	3,425	22,982	89,551	30,447
Other Comprehensive Income (Loss), After-Tax [Abstract]
Unrealized holding gains arising during period from debt securities available for sale			314,919	95,867
Less: reclassification adjustment for net gains on sale of debt securities in net income			29,149	(2,136)
Other comprehensive income, net of tax	10,933	73,950	285,770	98,003
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 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 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 gains arising during period from debt securities available for sale	41,651	112,339	172,594	180,107
Less: reclassification adjustment for net gains on sale of debt securities in net income	3,477	0	22,616	8,958
Other comprehensive income	38,174	112,339	149,978	171,149
Other Comprehensive Income (Loss), Tax Expense/(Benefit) [Abstract]
Unrealized holding gains arising during period from debt securities available for sale	9,936	26,635	41,180	42,703
Less: reclassification adjustment for net gains on sale of debt securities in net income	829	0	5,396	2,124
Other comprehensive income	9,107	26,635	35,784	40,579
Other Comprehensive Income (Loss), After-Tax [Abstract]
Unrealized holding gains arising during period from debt securities available for sale	31,715	85,704	131,414	137,404
Less: reclassification adjustment for net gains on sale of debt securities in net income	2,648	0	17,220	6,834
Other comprehensive income, net of tax	29,067	85,704	114,194	130,570
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	2,240	2,542	4,309	4,826
Other Comprehensive Income (Loss), Tax Expense/(Benefit) [Abstract]
Change in unamortized net holding gains on debt securities held to maturity	535	603	1,029	1,144
Other Comprehensive Income (Loss), After-Tax [Abstract]
Change in unamortized net holding losses on debt securities held to maturity	1,705	1,939	3,280	3,682
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	160	174	315	656
Other Comprehensive Income (Loss), Tax Expense/(Benefit) [Abstract]
Change in unamortized non-credit related impairment on debt securities held to maturity	39	42	76	156
Other Comprehensive Income (Loss), After-Tax [Abstract]
Change in unamortized non-credit related impairment on debt securities held to maturity	121	132	239	500
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	108	0	108
Other comprehensive income	2,400	2,608	4,624	5,374
Other Comprehensive Income (Loss), Tax Expense/(Benefit) [Abstract]
Less: reclassification adjustment for net gains on sale of debt securities in net income	0	26	0	26
Other comprehensive income	574	619	1,105	1,274
Other Comprehensive Income (Loss), After-Tax [Abstract]
Less: reclassification adjustment for net gains on sale of debt securities in net income	0	82	0	82
Other comprehensive income, net of tax	$ 1,826	$ 1,989	$ 3,519	$ 4,100

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