Long-term Incentive Compensation Plans (Details) - USD ($) $ in Thousands, shares in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Holding units purchased in period	0.1	1.3
Dollar amount paid for holding units acquired	$ 2,300	$ 31,000
Open-market purchases of holding units		1.2
Dollar amount paid for open-market purchases of holding units		$ 27,800
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Units issued upon options exercised (in units)	0.2	0.3
Proceeds from stock options exercised	$ 4,008	$ 4,468
Employees and Eligible Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted holding unit awards to employees (in units)	1.1	1.1