Debt (Schedule of Line of Credit Facilities) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 1,100,000,000	$ 1,335,000,000	$ 1,335,000,000
Borrowing base	605,927,000	734,464,000	773,228,000
Borrowings outstanding	196,500,000	565,000,000	541,500,000
Available borrowing capacity	409,427,000	169,464,000	231,728,000
Average borrowings outstanding	479,367,000	534,433,000	541,004,000
Borrowing Costs - interest	15,964,000	14,819,000	14,060,000
Borrowing Costs - fees	6,088,000	5,631,000	4,472,000
Interest expense	22,052,000	20,450,000	18,532,000
Agency Segment Credit Agreement
Line of Credit Facility [Line Items]
Maximum borrowing capacity	300,000,000	300,000,000	300,000,000
Borrowing base	148,299,000	165,443,000	225,642,000
Borrowings outstanding	0	0	0
Available borrowing capacity	148,299,000	165,443,000	225,642,000
Average borrowings outstanding	0	0	0
Borrowing Costs - interest	0	0	0
Borrowing Costs - fees	2,740,000	2,712,000	2,752,000
Interest expense	2,740,000	2,712,000	2,752,000
Finance Segment Credit Agreement
Line of Credit Facility [Line Items]
Maximum borrowing capacity	800,000,000	1,035,000,000	1,035,000,000
Borrowing base	457,628,000	569,021,000	547,586,000
Borrowings outstanding	196,500,000	565,000,000	541,500,000
Available borrowing capacity	261,128,000	4,021,000	6,086,000
Average borrowings outstanding	479,367,000	534,433,000	541,004,000
Borrowing Costs - interest	15,964,000	14,819,000	14,060,000
Borrowing Costs - fees	3,348,000	2,919,000	1,720,000
Interest expense	$ 19,312,000	$ 17,738,000	$ 15,780,000
Weighted average cost of borrowings (as a percent)	4.00%	3.30%	2.90%
Cost of finance revenues associated with reduction in borrowing capacity	$ 700,000