Long-Term Debt 5 1/2% Senior Notes (Details) - 5½% Senior Notes [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Sep. 09, 2013
Senior Notes [Member]
Debt Instrument, Face Amount		$ 300
Debt Instrument Offering Price Percentage		100.00%
Redemption Price Percent If Change In Control Occurs in Addition to Accrued and Unpaid Interest		101.00%
Debt Instrument, Redemption - After August 1, 2017
Debt Instrument, Redemption Price, Percentage in Addition to Accrued and Unpaid Interest	102.75%
Debt Instrument, Redemption - After Aug 1, 2019
Debt Instrument, Redemption Price, Percentage in Addition to Accrued and Unpaid Interest	100.00%