Investment Securities Maturity Schedule (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Available-for-Sale, Amortized Cost
Due within one year	$ 138,909
Due after one year through five years	471,592
Due after five years through ten years	134,959
Due after ten years	724,656
Total before residential mortgage-backed securities	1,470,116
Total	2,493,255		2,341,767
Available-for-Sale, Fair Value
Due within one year	139,512
Due after one year through five years	475,338
Due after five years through ten years	139,173
Due after ten years	753,513
Total before residential mortgage-backed securities	1,507,536
Total	2,544,093		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	2,081
Due after ten years	568,204
Total before residential mortgage-backed securities	570,285
Amortized Cost	570,285		520,997
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	2,092
Due after ten years	595,035
Total before residential mortgage-backed securities	597,127
Total	597,127		550,946
Residential mortgage-backed securities
Available-for-Sale, Amortized Cost
Residential mortgage-backed securities	1,023,139	[1]
Total	1,023,139		1,043,897
Available-for-Sale, Fair Value
Residential mortgage-backed securities	1,036,557	[1]
Total	1,036,557		1,052,616
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.