Investment Securities (Pooled Trust Preferred Collateralized Debt Obligations) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) loan entity	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2011 USD ($)
Fair Value, available for sale	$ 354,145		$ 277,378
Unrealized loss	(7,065)		$ (3,532)
Realized losses YTD	154	$ 129
Pooled Trust Preferred Securities [Member]
Original Par	4,000			$ 4,000
Book Value	3,923
Fair Value, available for sale	2,721
Unrealized loss	$ (1,201)
Pooled Trust Preferred Securities [Member] | Alesco Preferred Funding IX [Member]
Class	Aa3
Original Par	$ 1,000
Book Value	923
Fair Value, available for sale	599
Unrealized loss	$ (323)
Number of Banks / Insurance Cos. Currently Performing | loan	45
Total Number of Banks and Insurance Cos. In Issuance (Unique) | entity	48
Actual Deferrals/Defaults (as a % of original collateral)	2.85%
Total Projected Defaults (as a % of performing collateral)	8.23%
Excess subordination (after taking into account best estimate of future deferrals/defaults)	53.61%
Pooled Trust Preferred Securities [Member] | U.S. Capital Funding I [Member]
Class	B3
Original Par	$ 3,000
Book Value	3,000
Fair Value, available for sale	2,122
Unrealized loss	$ (878)
Number of Banks / Insurance Cos. Currently Performing | loan	26
Total Number of Banks and Insurance Cos. In Issuance (Unique) | entity	30
Actual Deferrals/Defaults (as a % of original collateral)	5.92%
Total Projected Defaults (as a % of performing collateral)	5.92%
Excess subordination (after taking into account best estimate of future deferrals/defaults)	12.29%
Minimum | Pooled Trust Preferred Securities [Member] | Alesco Preferred Funding IX [Member]
Class	CCC-
Minimum | Pooled Trust Preferred Securities [Member] | U.S. Capital Funding I [Member]
Class	Caa1