Summary of Accounting policies - Additional Information (Details)	3 Months Ended
Mar. 31, 2018 USD ($)
Summary of Accounting Policies [Abstract]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 1,498,000
Delay of Revenue Recognition Due To Implementation of New Standard	$ 407,000

Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant