Share-Based Compensation Performance Shares (Details) - Performance Shares $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares	Mar. 31, 2017 shares
Other disclosures
Vesting terms	3 years
Service period over which compensation expense would be amortized	3 years
Vested (in shares)		92
Award payout achieved (as a percent)		200
Cancelled (in shares)	4
Unrecognized compensation cost | $	$ 5.0
Remaining requisite service period (in years)	2 years 1 month 6 days
Minimum
Other disclosures
Performance shares available for vesting (as a percent)	0.00%
Maximum
Other disclosures
Performance shares available for vesting (as a percent)	200.00%
Long Term Incentive Plan
Other disclosures
Granted (in shares)	27
Granted (in dollars per share) | $ / shares	$ 141.91