Subsequent Events - Debt Obligations (Details) - USD ($)	1 Months Ended	3 Months Ended	9 Months Ended
	Oct. 31, 2015	Sep. 30, 2015	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Debt Obligations
Amount outstanding		$ 517,967,000	$ 517,967,000		$ 281,633,000
Senior Unsecured Debt
Debt Obligations
Amount outstanding		352,467,000	352,467,000		281,633,000
Senior Unsecured Debt | Note Purchase and Private Shelf Agreement AIG
Debt Obligations
Debt Instrument, Maximum Authorized for Issuance under Private Shelf Agreement		100,000,000
Senior Unsecured Debt | Subsequent Event | Note Purchase and Private Shelf Agreement AIG
Debt Obligations
Debt Instrument, Maximum Authorized for Issuance under Private Shelf Agreement	$ 100,000,000
Fixed interest rate (as a percent)	4.26%
Bank Borrowings - Line of Credit
Debt Obligations
Maximum availability under Unsecured Credit Agreement		400,000,000	400,000,000
Amount borrowed			267,000,000	$ 28,500,000
Amount outstanding		165,500,000	165,500,000
Remaining borrowing capacity		$ 234,500,000	$ 234,500,000		$ 400,000,000
Bank Borrowings - Line of Credit | Subsequent Event
Debt Obligations
Increase in maximum borrowing capacity due to exercise of accordion feature	$ 200,000,000
Maximum availability under Unsecured Credit Agreement	600,000,000
Amount borrowed	22,000,000
Amount outstanding	187,500,000
Remaining borrowing capacity	$ 412,500,000