Securities (Scheduled Maturities of Investment Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amortized Cost
Due within one year	$ 602,560
Due after one year through five years	772,349
Due after five years through ten years	220,298
Due after ten years	1,474,860
Total available-for-sale investment securities	3,070,067
Fair Value
Due within one year	583,126
Due after one year through five years	759,970
Due after five years through ten years	216,390
Due after ten years	1,457,266
Total available-for-sale investment securities	3,016,752	$ 3,335,795
Fair value of available-for-sale investment securities pledged	$ 534,300	$ 767,400