Securities - Investments in Pooled Trust Preferred Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Investments in pooled trust preferred securities
Amortized Cost	$ 674,104
Fair Value	671,488	$ 674,248
Trust preferred securities
Investments in pooled trust preferred securities
Amortized Cost	12,442	23,749
Fair Value	9,388	$ 18,389
Unrealized Loss	(3,054)
XXIII
Investments in pooled trust preferred securities
Amortized Cost	8,310
Fair Value	6,086
Unrealized Loss	$ (2,224)
Issuers currently in deferral or default (percent)	16.00%
XXVI
Investments in pooled trust preferred securities
Amortized Cost	$ 4,132
Fair Value	3,302
Unrealized Loss	$ (830)
Issuers currently in deferral or default (percent)	19.00%