Securities (Amortized Cost and Fair Value of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Debt Securities Available for Sale, Due in one year or less, Amortized Cost	$ 190
Debt Securities Available for Sale, Due after one year through five years, Amortized Cost	235,774
Debt Securities Available for Sale, Due after five years through ten years, Amortized Cost	1,178,941
Debt Securities Available for Sale, Due after ten years, Amortized Cost	1,343,818
Total available for sale debt securities, Amortized Cost	2,758,723
Debt Securities Available for Sale, Due in one year or less, Fair Value	194
Debt Securities Available for Sale, Due after one year through five years, Fair Value	239,806
Debt Securities Available for Sale, Due after five years through ten years, Fair Value	1,198,484
Debt Securities Available for Sale, Due after ten years, Fair Value	1,351,984
Total available for sale debt securities, Fair Value	2,790,468
Debt Securities Held to Maturity, Due in one year or less, Amortized Cost	62,394
Debt Securities Held to Maturity, Due after one year through five years, Amortized Cost	92,045
Debt Securities Held to Maturity, Due after five years through ten years, Amortized Cost	1,490,042
Debt Securities Held to Maturity, Due after ten years, Amortized Cost	1,290,786
Total held to maturity debt securities, Amortized Cost	2,935,267	$ 2,979,547
Debt Securities Held to Maturity, Due in one year or less, Fair Value	62,356
Debt Securities Held to Maturity, Due after one year through five years, Fair Value	93,495
Debt Securities Held to Maturity, Due after five years through ten years, Fair Value	1,525,907
Debt Securities Held to Maturity, Due after ten years, Fair Value	1,297,643
Total held to maturity debt securities, Fair Value	$ 2,979,401	$ 2,935,856