Commitments and contingencies (Debt Covenant Ratios) (Details)	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Fixed Charge Coverage Ratio	1.70	1.81	1.74	1.70	1.77	1.69	1.65	1.65	1.64	1.67	1.64	1.67
Leverage Ratio	4.07	3.83	4.02	4.05	3.80	4.02	4.05	4.12	4.21	4.08	4.40	4.80

Debt Instrument, Covenant Terms, Ratio of Indebtedness to Earnings Before Interest, Taxes, Depreciation and Amortization

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