Investment Securities Schedule of Amortized Cost and Fair Values of Debt Securities by Contractual Maturities (Details) $ in Thousands	Jun. 30, 2017 USD ($)
Amortized Cost
Due in one year or less	$ 20,103
Due from one year to five years	27,967
Due from five years to ten years	111,883
Due after ten years	447,339
Amortized cost, before securities without debt maturities	607,292
Amortized Cost	2,493,408
Estimated Fair Value
Due in one year or less	20,336
Due from one year to five years	28,335
Due from five years to ten years	113,050
Due after ten years	432,884
Available for sale securities, debt maturities, before securities without single maturities	594,605
Estimated Fair Value	2,467,244
Collateralized Mortgage Obligations [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	546,847
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	538,507
Commercial Mortgage Backed Securities [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	121,012
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	121,067
Mortgage-Backed Securities [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	1,218,257
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	$ 1,213,065