Consolidated Statements of Retained Earnings - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement Of Partners Capital [Abstract]
Beginning Balance	$ 313.7	$ 305.8	$ 308.1
Add: Net income	285.7	276.3	260.3
Retained Earning, Gross	599.4	582.1	568.4
Deduct: Cash dividends on capital stock—common	288.2	268.4	262.6
Ending Balance	$ 311.2	$ 313.7	$ 305.8