Debt Securities Maturity Schedule (Details) - USD ($) $ in Thousands		Mar. 31, 2020	Dec. 31, 2019
Available-for-Sale, Amortized Cost
Due within one year		$ 125,320
Due after one year through five years		280,574
Due after five years through ten years		241,013
Due after ten years		958,111
Total before mortgage-backed securities		1,605,018
Total		3,296,319	$ 2,521,374
Available-for-Sale, Fair Value
Due within one year		125,118
Due after one year through five years		282,188
Due after five years through ten years		250,182
Due after ten years		1,007,856
Total before mortgage-backed securities		1,665,344
Debt securities, available-for-sale		3,429,890	2,575,252
Held-to-Maturity, Amortized Cost
Due within one year		0
Due after one year through five years		12,153
Due after five years through ten years		72,466
Due after ten years		119,195
Total before mortgage-backed securities		203,814
Amortized Cost		203,814	224,611
Held-to-Maturity, Fair Value
Due within one year		0
Due after one year through five years		12,637
Due after five years through ten years		76,544
Due after ten years		124,849
Total before mortgage-backed securities		214,030
Total		214,030	$ 234,396
Mortgage-backed securities
Available-for-Sale, Amortized Cost
Mortgage-backed securities	[1]	1,691,301
Available-for-Sale, Fair Value
Mortgage-backed securities	[1]	1,764,546
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.