Employee Benefits, Summary of Activity in LTIP Incentive Compensation Plan (Details) - LTIP - Phantom Share Units (PSUs) $ / shares in Units, $ in Millions		12 Months Ended
		Dec. 31, 2017 USD ($) yr $ / shares shares	Dec. 31, 2016 USD ($) yr $ / shares shares	Dec. 31, 2015 USD ($) yr shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding balance, beginning (in units)		1,257,625	645,968
Granted (in units)		487,142	865,091
Paid (in units)		(735,231)	(237,972)
Forfeited (in units)		(36,641)	(15,462)
Outstanding balance, ending (in units)		972,895	1,257,625	645,968
Unit Appreciation Rights and All Unit-Based Incentive Compensation Plans, Units, Fair Value [Abstract]
Outstanding balance, beginning | $	[1]	$ 22.5	$ 8.7
Granted | $		8.1	10.2
Paid | $		(11.2)	(4.1)
Forfeited | $		0.0	0.0
Outstanding balance, ending | $	[1]	$ 13.1	$ 22.5	$ 8.7
Unit Appreciation Rights and All Unit-Based Incentive Compensation Plans, Units, Weighted-Average Vesting Period [Abstract]
Outstanding balance, beginning | yr	[1]	1.2	1.5
Granted | yr		2.3	2.3
Outstanding balance, ending | yr	[1]	1.0	1.2	1.5
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized		3,525,000
Closing market price of the common unit | $ / shares		$ 12.91	$ 17.36
Compensation expense | $		$ 7.8	$ 11.6	$ 3.6
Total estimated remaining unrecognized compensation expense related to the Phantom Common Units | $		$ 6.0	$ 11.9
Share-based Compensation Arrangement by Share-based Payment Award, Shares Purchased for Award		10,812	17,108
Share-based Compensation Arrangement by Share-based Payment Award, Per Share Weighted Average Price of Shares Purchased | $ / shares		$ 18.50	$ 11.75
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant		3,450,060

[1] Represents fair value and remaining weighted-average vesting period of outstanding awards at the end of the period.