Debt Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Available-for-sale
Amortized cost	$ 5,145,866	$ 2,521,374
Gross unrealized gains	192,740	55,965
Gross unrealized losses	(792)	(2,087)
Total	5,337,814	2,575,252
Held-to-maturity
Amortized cost	189,836	224,611
Gross unrealized gains	13,380	9,785
Gross unrealized losses	0	0
Fair value	203,216	234,396
Debt Securities, Available-for-sale and Held-to-maturity [Abstract]
Amortized cost	5,335,702	2,745,985
Gross unrealized gains	206,120	65,750
Gross unrealized losses	(792)	(2,087)
Fair value	5,541,030	2,809,648
U.S. government and federal agency
Available-for-sale
Amortized cost	38,568	20,061
Gross unrealized gains	287	48
Gross unrealized losses	(267)	(65)
Total	38,588	20,044
U.S. government sponsored enterprises
Available-for-sale
Amortized cost	9,747	42,724
Gross unrealized gains	34	953
Gross unrealized losses	0	0
Total	9,781	43,677
State and local governments
Available-for-sale
Amortized cost	1,321,763	679,784
Gross unrealized gains	94,974	22,694
Gross unrealized losses	(54)	(80)
Total	1,416,683	702,398
Held-to-maturity
Amortized cost	189,836	224,611
Gross unrealized gains	13,380	9,785
Gross unrealized losses	0	0
Fair value	203,216	234,396
Corporate bonds
Available-for-sale
Amortized cost	336,867	155,665
Gross unrealized gains	12,239	1,938
Gross unrealized losses	(8)	(1)
Total	349,098	157,602
Residential mortgage-backed securities
Available-for-sale
Amortized cost	2,261,463	731,766
Gross unrealized gains	27,631	7,507
Gross unrealized losses	(4)	(549)
Total	2,289,090	738,724
Commercial mortgage-backed securities
Available-for-sale
Amortized cost	1,177,458	891,374
Gross unrealized gains	57,575	22,825
Gross unrealized losses	(459)	(1,392)
Total	$ 1,234,574	$ 912,807