Debt Securities - Maturity Schedule (Details) - USD ($) $ in Thousands		Sep. 30, 2020	Dec. 31, 2019
Available-for-Sale, Amortized Cost
Due within one year		$ 119,874
Due after one year through five years		264,927
Due after five years through ten years		263,208
Due after ten years		942,145
Total before mortgage-backed securities		1,590,154
Total		3,949,422	$ 2,521,374
Available-for-Sale, Fair Value
Due within one year		121,303
Due after one year through five years		277,758
Due after five years through ten years		277,022
Due after ten years		1,010,282
Total before mortgage-backed securities		1,686,365
Debt securities, available-for-sale		4,125,548	2,575,252
Held-to-Maturity, Amortized Cost
Due within one year		0
Due after one year through five years		19,543
Due after five years through ten years		69,924
Due after ten years		104,042
Total before mortgage-backed securities		193,509
Total		193,509	224,611
Held-to-Maturity, Fair Value
Due within one year		0
Due after one year through five years		20,996
Due after five years through ten years		75,680
Due after ten years		109,901
Total before mortgage-backed securities		206,577
Total		206,577	$ 234,396
Mortgage-backed securities
Available-for-Sale, Amortized Cost
Mortgage-backed securities	[1]	2,359,268
Available-for-Sale, Fair Value
Mortgage-backed securities	[1]	2,439,183
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.