Debt Securities Maturity Schedule (Details) - USD ($) $ in Thousands		Sep. 30, 2019	Dec. 31, 2018
Available-for-Sale, Amortized Cost
Due within one year		$ 160,855
Due after one year through five years		209,725
Due after five years through ten years		215,297
Due after ten years		361,605
Total before mortgage-backed securities		947,482
Total		2,394,617	$ 2,580,520
Available-for-Sale, Fair Value
Due within one year		160,980
Due after one year through five years		213,094
Due after five years through ten years		224,814
Due after ten years		376,983
Total before mortgage-backed securities		975,871
Debt securities, available-for-sale		2,459,036	2,571,663
Held-to-Maturity, Amortized Cost
Due within one year		0
Due after one year through five years		10,204
Due after five years through ten years		75,545
Due after ten years		149,243
Total before mortgage-backed securities		234,992
Amortized Cost		234,992	297,915
Held-to-Maturity, Fair Value
Due within one year		0
Due after one year through five years		10,580
Due after five years through ten years		79,647
Due after ten years		155,332
Total before mortgage-backed securities		245,559
Total		245,559	$ 288,256
Mortgage-backed securities
Available-for-Sale, Amortized Cost
Mortgage-backed securities	[1]	1,447,135
Available-for-Sale, Fair Value
Mortgage-backed securities	[1]	1,483,165
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.