Long-Term Debt - Credit Facilities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Debt Disclosure [Abstract]
Credit facility outstanding	$ 131,500		$ 131,500		$ 0
Available for borrowings	918,500		918,500		1,050,000
Total borrowing commitments	1,050,000		1,050,000		$ 1,050,000
Maximum amount outstanding at any month-end	131,500	$ 0	131,500	$ 214,500
Average daily amount outstanding	$ 47,484	$ 374	$ 23,887	$ 67,654
Average daily amount outstanding	2.38%	1.72%	2.38%	1.72%
Amount outstanding at period-end	2.44%	0.00%	2.44%	0.00%