Bonds and Notes Payable - Debt Repurchases (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Par value	$ 184,210	$ 85,412	$ 46,530
Purchase price	181,308	77,431	41,728
Gain (loss) from debt repurchases	2,902	7,981	4,802
Asset-backed Securities [Member]
Debt Instrument [Line Items]
Par value	154,407	78,412	32,026
Purchase price	155,951	72,566	30,354
Gain (loss) from debt repurchases	(1,544)	5,846	1,672
Unsecured Debt - Hybrid Securities [Member]
Debt Instrument [Line Items]
Par value	29,803	7,000	14,504
Purchase price	25,357	4,865	11,374
Gain (loss) from debt repurchases	$ 4,446	$ 2,135	$ 3,130