Debt Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Available-for-sale
Amortized Cost	$ 6,636,833	$ 9,175,423
Gross Unrealized Gains	4,673	68,117
Gross Unrealized Losses	(432,307)	(72,691)
Fair Value	6,209,199	9,170,849
Held-to-maturity
Amortized Cost	3,788,486	1,199,164
Gross Unrealized Gains	990	22,878
Gross Unrealized Losses	(329,717)	(1,159)
Fair Value	3,459,759	1,220,883
Total debt securities
Amortized Cost	10,425,319	10,374,587
Gross Unrealized Gains	5,663	90,995
Gross Unrealized Losses	(762,024)	(73,850)
Fair Value	9,668,958	10,391,732
U.S. government and federal agency
Available-for-sale
Amortized Cost	489,798	1,356,171
Gross Unrealized Gains	93	174
Gross Unrealized Losses	(32,007)	(9,596)
Fair Value	457,884	1,346,749
Held-to-maturity
Amortized Cost	844,175
Gross Unrealized Gains	0
Gross Unrealized Losses	(56,566)
Fair Value	787,609
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	319,509	241,687
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(21,695)	(996)
Fair Value	297,814	240,693
State and local governments
Available-for-sale
Amortized Cost	440,473	461,414
Gross Unrealized Gains	2,794	27,567
Gross Unrealized Losses	(5,650)	(123)
Fair Value	437,617	488,858
Held-to-maturity
Amortized Cost	1,653,376	1,199,164
Gross Unrealized Gains	990	22,878
Gross Unrealized Losses	(215,836)	(1,159)
Fair Value	1,438,530	1,220,883
Corporate bonds
Available-for-sale
Amortized Cost	101,153	175,697
Gross Unrealized Gains	677	5,072
Gross Unrealized Losses	(482)	(17)
Fair Value	101,348	180,752
Residential mortgage-backed securities
Available-for-sale
Amortized Cost	4,025,432	5,744,505
Gross Unrealized Gains	187	9,420
Gross Unrealized Losses	(310,117)	(54,266)
Fair Value	3,715,502	5,699,659
Held-to-maturity
Amortized Cost	1,290,935
Gross Unrealized Gains	0
Gross Unrealized Losses	(57,315)
Fair Value	1,233,620
Commercial mortgage-backed securities
Available-for-sale
Amortized Cost	1,260,468	1,195,949
Gross Unrealized Gains	922	25,882
Gross Unrealized Losses	(62,356)	(7,693)
Fair Value	$ 1,199,034	$ 1,214,138