Securities Held-to-Maturity (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) item	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Held-to-Maturity
Carrying Value	$ 36,764		$ 27,837
Held-to-Maturity | B Piece bonds
Held-to-Maturity
Initial face value of bonds purchased	12,300
Discounted value of bonds purchased	$ 8,400
B Piece retained percentage	49.00%
Number of B Piece bonds retained | item	4
Discount value	$ 35,600
Remaining of B Piece bond sold to the third party at par	51.00%
Weighted average variable interest rate (as a percent)	3.64%
Estimated weighted average maturity period	5 years 8 months 12 days
Weighted average effective interest rate (as a percent)	11.43%		12.97%
Face Value	$ 52,673		$ 40,566
Carrying Value	36,764		27,837
Unrealized Gain	509		602
Estimated Fair Value	37,273		$ 28,439
Impairment charges	0
Interest income	600	$ 100
Held-to-maturity securities, estimated fiscal year
Within one year	8,000
After one year through five years	25,400
After five years through ten years	12,300
After ten years	$ 7,000