Investment Securities Investment Securities Maturity Schedule (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Available-for-Sale, Amortized Cost
Due within one year	$ 126,102
Due after one year through five years	367,037
Due after five years through ten years	90,172
Due after ten years	714,559
Total before residential mortgage-backed securities	1,297,870
Total	2,341,767
Available-for-Sale, Fair Value
Due with one year	126,779
Due after one year through five years	370,349
Due after five years through ten years	93,356
Due after ten years	744,328
Total before residential mortgage-backed securities	1,334,812
Total	2,387,428
Held-to-Maturity, Amortized Cost
Due within one year	0
Due after one year through five years	0
Due after five years through ten years	188
Due after ten years	520,809
Total before residential mortgage-backed securities	520,997
Total	520,997		0
Held-to-Maturity, Fair Value
Due within one year	0
Due after one year through five years	0
Due after five years through ten years	188
Due after ten years	550,758
Total before residential mortgage-backed securities	550,946
Total	550,946
Residential mortgage-backed securities
Available-for-Sale, Amortized Cost
Residential mortgage-backed securities	1,043,897	[1]
Available-for-Sale, Fair Value
Residential mortgage-backed securities	1,052,616	[1]
Held-to-Maturity, Amortized Cost
Residential mortgage-backed securities	0	[1]
Held-to-Maturity, Fair Value
Residential mortgage-backed securities	$ 0	[1]

[1] Residential mortgage-backed securities, which have prepayment provisions, are not assigned to maturity categories due to fluctuations in their prepayment speeds.