Investment Securities (Summary of the Amortized Cost and Fair Value of Investment Securities by Maturity) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less	$ 51,523
Due after one year through five years	109,918
Due after five years through ten years	515,050
Due after ten years	2,393,154
Total	3,069,645	$ 1,975,257
Fair Value
Due in one year or less	51,244
Due after one year through five years	108,051
Due after five years through ten years	498,612
Due after ten years	2,363,925
Total	3,021,832	1,961,722
Amortized Cost
Due in one year or less	55,840
Due after one year through five years	230,154
Due after five years through ten years	164,325
Due after ten years	173,780
Total	624,099	677,093
Fair Value
Due in one year or less	55,728
Due after one year through five years	228,528
Due after five years through ten years	162,764
Due after ten years	171,652
Total	$ 618,672	$ 662,906