Long-Term Debt - Credit Facilities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Debt Disclosure [Abstract]
Credit facility outstanding	$ 301,000		$ 301,000		$ 255,000
Available for borrowings	949,000		949,000		795,000
Total borrowing commitments	1,250,000		1,250,000		$ 1,050,000
Maximum amount outstanding at any month-end	301,000	$ 131,500	301,000	$ 131,500
Average daily amount outstanding	$ 290,876	$ 47,484	$ 270,139	$ 23,887
Average daily amount outstanding	3.23%	2.38%	3.08%	2.38%
Amount outstanding at period-end	3.32%	2.44%	3.32%	2.44%