Debt - Senior Unsecured Notes (Details) - Senior notes - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Series D, Series E, and Series F Senior Unsecured Notes
Debt Instrument [Line Items]
Face amount		$ 450,000,000.0
Senior unsecured notes, Series D
Debt Instrument [Line Items]
Face amount		$ 115,000,000.0
Stated Rate (as a percent)	4.08%	4.08%
Senior unsecured notes, Series E
Debt Instrument [Line Items]
Face amount		$ 160,000,000.0
Stated Rate (as a percent)	4.26%	4.26%
Senior unsecured notes, Series F
Debt Instrument [Line Items]
Face amount		$ 175,000,000.0
Stated Rate (as a percent)	4.44%	4.44%