Debt Obligations - Summary (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Debt Obligations
Applicable Interest Rate (as a percent)		4.11%
Outstanding Balance		$ 609,391,000	$ 571,872,000
Repayment amount		137,000,000	170,500,000	$ 58,500,000
Proceeds from issuance of senior unsecured notes		$ 77,500,000	200,000,000	$ 30,000,000
Bank Borrowings - Line of Credit
Debt Obligations
Applicable Interest Rate (as a percent)		2.25%
Outstanding Balance		$ 107,100,000	120,500,000
Available for borrowing		492,900,000	479,500,000
Repayment amount		$ 137,000,000	170,500,000
Bank Borrowings - Line of Credit | Subsequent Event
Debt Obligations
Outstanding Balance	$ 0
Available for borrowing	600,000,000
Repayment amount	107,100,000
Senior Unsecured Notes
Debt Obligations
Applicable Interest Rate (as a percent)		4.50%
Outstanding Balance		$ 502,291,000	451,372,000
Available for borrowing		22,500,000	33,333,000
Senior Unsecured Notes | Private Shelf Agreement Prudential
Debt Obligations
Available for borrowing		22,500,000
Repayments of Debt			29,167,000
Face amount of debt		$ 37,500,000	$ 100,000,000
Senior Unsecured Notes | Private Shelf Agreement Prudential | Subsequent Event
Debt Obligations
Outstanding Balance	598,124,000
Available for borrowing	36,667,000
Repayments of Debt	4,167,000
Face amount of debt	$ 100,000,000