Investment Securities Schedule of Amortized Cost and Fair Values of Debt Securities by Contractual Maturities (Details) $ in Thousands	Jun. 30, 2018 USD ($)
Amortized Cost
Due in one year or less	$ 7,305
Due from one year to five years	44,956
Due from five years to ten years	109,639
Due after ten years	480,211
Amortized cost, before securities without debt maturities	642,111
Amortized Cost	2,664,163
Estimated Fair Value
Due in one year or less	7,313
Due from one year to five years	44,959
Due from five years to ten years	109,503
Due after ten years	467,439
Available for sale securities, debt maturities, before securities without single maturities	629,214
Estimated Fair Value	2,593,283
Collateralized Mortgage Obligations [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	742,282
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	724,322
Commercial Mortgage Backed Securities [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	244,046
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	238,783
Mortgage-Backed Securities [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	1,035,724
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	$ 1,000,964