Securities (Change in Credit Loss Component of Credit Impaired Debt Securities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Credit loss component, beginning of period	$ 0	$ 1,750	$ 981
Additions:
Initial credit impairments	0	0	0
Subsequent credit impairments	0	0	769
Total additions	0	0	769
Reductions:
For securities sold	0	1,750	0
Total reductions	0	1,750	0
Credit loss component, end of period	$ 0	$ 0	$ 1,750