Employee Benefits, Valuation Assumptions Under Unit Appreciation Rights (Details) - Long-Term Cash Bonus Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred Compensation Arrangement with Individual, Cash Award Granted, Amount	$ 2.7
Cash Bonus Granted During Period				$ 9.2
Deferred Compensation Arrangement with Individual, Compensation Expense	1.1	$ 3.5	$ 2.8
Unrecognized compensation cost	$ 1.6