Revenue recognition (Details) $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($) Period Segment
Revenue [Abstract]
Number of operating segments \| Segment	1
CIADM [Member]
Revenue [Abstract]
Term of contract	24 years
Number of additional option periods \| Period	17
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] \| CIADM [Member] \| Accounting Standards Update 2014-09 [Member]
Revenue [Abstract]
Retained earnings	$ 32.5
Deferred tax assets	$ 9.9

Amount of cumulative effect of the change in accounting principle or new accounting pronouncement on retained earnings or other components of equity or net assets in the statement of financial position as of the beginning of the earliest period presented.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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