Securities (Scheduled Maturities of Investment Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized Cost
Due within one year	$ 573,138
Due after one year through five years	818,412
Due after five years through ten years	188,149
Due after ten years	1,311,194
Total available-for-sale investment securities	2,890,893
Fair Value
Due within one year	549,190
Due after one year through five years	796,717
Due after five years through ten years	184,944
Due after ten years	1,280,565
Total available-for-sale investment securities	2,811,416	$ 3,016,752
Fair values of available-for-sale investment securities pledged	$ 498,700	$ 534,300