Long-term Incentive Compensation Plans (Details) - USD ($) $ in Thousands, shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Holding units purchased in period	0.3	2.0	5.9	5.8
Dollar amount paid for holding units acquired	$ 6,900	$ 45,200	$ 134,600	$ 129,200
Open-market purchases of holding units	0.3	2.0	5.2	5.7
Dollar amount paid for open-market purchases of holding units	$ 6,800	$ 45,100	$ 117,100	$ 127,100
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Units issued upon options exercised (in units)			1.0	0.1
Proceeds from stock options exercised			$ 17,672	$ 2,371
Employees and Eligible Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted holding unit awards to employees (in units)			2.2	0.7