Debt - Public Debt (Details) - 5.72% Notes, due 2019 - USD ($)	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term debt, interest rate	5.72%	5.72%
Long-term debt	$ 500,000,000
Redemption amount	518,000,000
Loss on prepayment of debt redemption	$ 19,000,000