Securities (Scheduled Maturities of Investment Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Amortized Cost
Due within one year	$ 632,583
Due after one year through five years	697,172
Due after five years through ten years	199,008
Due after ten years	1,270,710
Total available-for-sale investment securities	2,799,473
Fair Value
Due within one year	603,002
Due after one year through five years	675,324
Due after five years through ten years	194,040
Due after ten years	1,235,078
Total available-for-sale investment securities	2,707,444	$ 3,016,752
Fair values of available-for-sale investment securities pledged	$ 436,800	$ 534,300