Securities (Realized Gains and Losses) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Proceeds from sales	$ 832,844	$ 1,275,645	$ 1,669,334
Gross realized gains	8,037	10,487	40,367
Gross realized losses	0	125	0
Related tax expense	$ 3,380	$ 4,357	$ 16,974