Borrowings Under Credit Facilities and Commercial Paper Program - Aggregate Borrowings Under Unsecured Revolving Credit Facility and Commercial Paper (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	[1]
Debt Disclosure [Abstract]
Balance outstanding at quarter end	$ 419,293	$ 865,000	$ 1,147,000
Maximum amount outstanding at any month end	1,150,000	865,000
Average amount outstanding (total of daily principal balances divided by days in period)	$ 790,516	$ 364,111
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	3.22%	2.72%

[1] The consolidated balance sheet at December 31, 2018 has been derived from the audited financial statements at that date but does not include all of the information and footnotes required by U.S. generally accepted accounting principles for complete financial statements.