Long-Term Debt - Credit Facilities (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Apr. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Credit facility outstanding	$ 228,000,000			$ 255,000,000
Available for borrowings	822,000,000			795,000,000
Total borrowing commitments	1,050,000,000			$ 1,050,000,000
Maximum amount outstanding at any month-end	240,000,000	$ 0
Average daily amount outstanding	$ 249,172,000	$ 28,000
Average daily amount outstanding	2.89%	2.05%
Amount outstanding at period-end	3.09%	0.00%
Credit facility
Line of Credit Facility [Line Items]
Total borrowing commitments	$ 1,050,000,000
Credit facility | Subsequent Event | Syndicated Credit Facility
Line of Credit Facility [Line Items]
Total borrowing commitments			$ 1,250,000,000