Debt - Senior Unsecured Notes (Details) - Senior Notes [Member]	Dec. 31, 2017 USD ($)
Series D, Series E, and Series F Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Face amount	$ 450,000,000.0
Series D Senior Notes due 2028 [Member]
Debt Instrument [Line Items]
Face amount	$ 115,000,000.0
Stated interest rate	4.08%
Series E Senior Notes due 2030 [Member]
Debt Instrument [Line Items]
Face amount	$ 160,000,000.0
Stated interest rate	4.26%
Series F Senior Notes due 2033 [Member]
Debt Instrument [Line Items]
Face amount	$ 175,000,000.0
Stated interest rate	4.44%