Debt Securities Maturity Schedule (Details) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017
Available-for-Sale, Amortized Cost
Due within one year		$ 141,771
Due after one year through five years		316,311
Due after five years through ten years		311,194
Due after ten years		511,839
Total before mortgage-backed securities		1,281,115
Amortized Cost		2,580,520	$ 1,771,504
Available-for-Sale, Fair Value
Due with one year		141,356
Due after one year through five years		314,777
Due after five years through ten years		314,961
Due after ten years		515,830
Total before mortgage-backed securities		1,286,924
Total		2,571,663	1,778,243
Held-to-Maturity, Amortized Cost
Due within one year		0
Due after one year through five years		6,457
Due after five years through ten years		75,204
Due after ten years		216,254
Total before mortgage-backed securities		297,915
Amortized Cost		297,915	648,313
Held-to-Maturity, Fair Value
Due within one year		0
Due after one year through five years		6,527
Due after five years through ten years		74,100
Due after ten years		207,629
Total before mortgage-backed securities		288,256
Total		288,256	$ 660,086
Mortgage-backed securities
Available-for-Sale, Amortized Cost
Mortgage-backed securities	[1]	1,299,405
Available-for-Sale, Fair Value
Mortgage-backed securities	[1]	1,284,739
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.