Debt - Unsecured Credit Facility (Details) - Unsecured credit facility - USD ($)	1 Months Ended
	Dec. 31, 2018	Jan. 31, 2019	Dec. 31, 2017
Line of Credit Facility [Line Items]
Face amount	$ 500,000,000
Short-term borrowing	$ 250,000,000		$ 0
Number of increases to margin	2
Percentage increase to margin	0.50%
Interest rate on outstanding borrowings	6.01%
LIBOR
Line of Credit Facility [Line Items]
Applicable margin on variable rate	3.50%
Base Rate
Line of Credit Facility [Line Items]
Applicable margin on variable rate	2.50%
Subsequent Event
Line of Credit Facility [Line Items]
Short-term borrowing		$ 250,000,000