Senior Notes, Secured, and Unsecured Indebtedness - Subordinated Amortizing Notes - Narrative (Details) $ / shares in Units, $ in Thousands	Nov. 21, 2014 $ / shares $ / note shares	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 03, 2014 shares
Debt Instrument [Line Items]
Total debt | $		$ 1,097,633	$ 1,080,650
Subordinated amortizing notes
Debt Instrument [Line Items]
Initial principal amount | $ / note	18.01
Stated interest rate	5.50%
Quarterly installments amount per amortizing note (in USD per note) | $ / note	1.6250
Installment payment amount per amortizing note (in USD per note) | $ / note	1.8056
Total debt | $		1,619	7,225
Senior unsecured loan facility
Debt Instrument [Line Items]
Total debt | $		$ 1,619	$ 7,225
Minimum | Common stock, Class A
Debt Instrument [Line Items]
Shares delivered on mandatory settlement date (in shares) | shares	4.4465
Maximum | Common stock, Class A
Debt Instrument [Line Items]
Shares delivered on mandatory settlement date (in shares) | shares	5.2247
Tangible Equity Units
Debt Instrument [Line Items]
Number of TEUs sold (in shares) | shares	1,000,000			150,000
Stated rate	6.50%
Share price (in USD per share) | $ / shares	$ 100
Percent conversion premium	17.50%