Debt Securities - Maturity Schedule (Details) - USD ($) $ in Thousands		Dec. 31, 2020	Dec. 31, 2019
Available-for-Sale, Amortized Cost
Due within one year		$ 128,263
Due after one year through five years		246,473
Due after five years through ten years		266,248
Due after ten years		1,065,961
Total before mortgage-backed securities		1,706,945
Amortized cost		5,145,866	$ 2,521,374
Available-for-Sale, Fair Value
Due within one year		129,346
Due after one year through five years		258,040
Due after five years through ten years		280,021
Due after ten years		1,146,743
Total before mortgage-backed securities		1,814,150
Total		5,337,814	2,575,252
Held-to-Maturity, Amortized Cost
Due within one year		484
Due after one year through five years		22,032
Due after five years through ten years		67,678
Due after ten years		99,642
Total before mortgage-backed securities		189,836
Amortized cost		189,836	224,611
Held-to-Maturity, Fair Value
Due within one year		489
Due after one year through five years		23,625
Due after five years through ten years		73,367
Due after ten years		105,735
Total before mortgage-backed securities		203,216
Total		203,216	$ 234,396
Mortgage-backed securities
Available-for-Sale, Amortized Cost
Mortgage-backed securities	[1]	3,438,921
Available-for-Sale, Fair Value
Mortgage-backed securities	[1]	3,523,664
Held-to-Maturity, Amortized Cost
Mortgage-backed securities	[1]	0
Held-to-Maturity, Fair Value
Mortgage-backed securities	[1]	$ 0

[1] Mortgage-backed securities, which have prepayment provisions, are not assigned to maturity categories due to fluctuations in their prepayment speeds.