Investment Securities Schedule of Amortized Cost and Fair Values of Debt Securities by Contractual Maturities (Details) $ in Thousands	Mar. 31, 2016 USD ($)
Amortized Cost
Due in one year or less	$ 64,096
Due from one year to five years	89,237
Due from five years to ten years	92,154
Due after ten years	288,960
Amortized cost, before securities without debt maturities	534,447
Amortized Cost	2,486,017
Estimated Fair Value
Due in one year or less	64,475
Due from one year to five years	91,302
Due from five years to ten years	95,021
Due after ten years	275,597
Available for sale securities, debt maturities, before securities without single maturities	526,395
Estimated Fair Value	2,497,138
Collateralized Mortgage Obligations [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	782,018
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	782,075
Mortgage-Backed Securities [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	1,169,552
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	$ 1,188,668