Investment Securities Summary of Cumulative Other-than-Temporary Impairment Charges Recognized in Earnings for Pooled Trust Preferred Securities Held (Details) - Pooled trust preferred securities - 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]
Balance of cumulative credit losses on debt securities, beginning of year	$ (11,510)	$ (11,510)	$ (16,242)
Reductions for securities sold during the period	0	0	4,730
Reductions for increases in cash flows expected to be collected that are recognized over the remaining life of the security	0	0	2
Balance of cumulative credit losses on debt securities, end of year	$ (11,510)	$ (11,510)	$ (11,510)