Debt Securities Maturity Schedule (Details) - USD ($) $ in Thousands		Mar. 31, 2019	Dec. 31, 2018
Available-for-Sale, Amortized Cost
Due within one year		$ 88,040
Due after one year through five years		243,020
Due after five years through ten years		272,600
Due after ten years		440,778
Total before mortgage-backed securities		1,044,438
Total		2,484,948	$ 2,580,520
Available-for-Sale, Fair Value
Due within one year		88,011
Due after one year through five years		245,035
Due after five years through ten years		282,663
Due after ten years		456,008
Total before mortgage-backed securities		1,071,717
Debt securities, available-for-sale		2,522,322	2,571,663
Held-to-Maturity, Amortized Cost
Due within one year		0
Due after one year through five years		7,061
Due after five years through ten years		76,025
Due after ten years		172,486
Total before mortgage-backed securities		255,572
Amortized Cost		255,572	297,915
Held-to-Maturity, Fair Value
Due within one year		0
Due after one year through five years		7,230
Due after five years through ten years		78,561
Due after ten years		175,617
Total before mortgage-backed securities		261,408
Total		261,408	$ 288,256
Mortgage-backed securities
Available-for-Sale, Amortized Cost
Mortgage-backed securities	[1]	1,440,510
Available-for-Sale, Fair Value
Mortgage-backed securities	[1]	1,450,605
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.