Debt Securities - Maturity Schedule (Details) - USD ($) $ in Thousands		Mar. 31, 2021	Dec. 31, 2020
Available-for-Sale, Amortized Cost
Due within one year		$ 91,067
Due after one year through five years		237,676
Due after five years through ten years		247,854
Due after ten years		658,242
Total before mortgage-backed securities		1,234,839
Total		5,750,190	$ 5,145,866
Available-for-Sale, Fair Value
Due within one year		92,128
Due after one year through five years		248,388
Due after five years through ten years		257,032
Due after ten years		711,001
Total before mortgage-backed securities		1,308,549
Debt securities, available-for-sale		5,853,315	5,337,814
Held-to-Maturity, Amortized Cost
Due within one year		482
Due after one year through five years		23,431
Due after five years through ten years		66,883
Due after ten years		497,955
Total before mortgage-backed securities		588,751
Total		588,751	189,836
Held-to-Maturity, Fair Value
Due within one year		486
Due after one year through five years		24,883
Due after five years through ten years		71,375
Due after ten years		502,216
Total before mortgage-backed securities		598,960
Total		598,960	$ 203,216
Mortgage-backed securities
Available-for-Sale, Amortized Cost
Mortgage-backed securities	[1]	4,515,351
Available-for-Sale, Fair Value
Mortgage-backed securities	[1]	4,544,766
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.