Securities (Amortized Cost and Fair Value of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Debt Securities Available for Sale, Due in one year or less, Amortized Cost	$ 368
Debt Securities Available for Sale, Due after one year through five years, Amortized Cost	107,225
Debt Securities Available for Sale, Due after five years through ten years, Amortized Cost	1,184,905
Debt Securities Available for Sale, Due after ten years, Amortized Cost	1,406,367
Total available for sale debt securities, Amortized Cost	2,698,865
Debt Securities Available for Sale, Due in one year or less, Fair Value	374
Debt Securities Available for Sale, Due after one year through five years, Fair Value	108,866
Debt Securities Available for Sale, Due after five years through ten years, Fair Value	1,175,070
Debt Securities Available for Sale, Due after ten years, Fair Value	1,396,770
Total available for sale debt securities, Fair Value	2,681,080
Debt Securities Held to Maturity, Due in one year or less, Amortized Cost	80,915
Debt Securities Held to Maturity, Due after one year through five years, Amortized Cost	67,617
Debt Securities Held to Maturity, Due after five years through ten years, Amortized Cost	1,392,654
Debt Securities Held to Maturity, Due after ten years, Amortized Cost	1,355,256
Total held to maturity debt securities, Amortized Cost	2,896,442	$ 2,979,547
Debt Securities Held to Maturity, Due in one year or less, Fair Value	80,716
Debt Securities Held to Maturity, Due after one year through five years, Fair Value	67,418
Debt Securities Held to Maturity, Due after five years through ten years, Fair Value	1,398,880
Debt Securities Held to Maturity, Due after ten years, Fair Value	1,345,896
Total held to maturity debt securities, Fair Value	$ 2,892,910	$ 2,935,856