Leases - Lease cost components (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2019
Lease cost components
Amortization of leased asset	$ 1.4	$ 1.9
Interest on lease liabilities	0.2	0.2
Operating lease cost	2.5	4.4
Net lease cost	4.1	6.5
Short-term lease and variable lease costs	$ 0.5	$ 0.5

Amount of interest expense on finance lease liability.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918686-209980

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918638-209977

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 55
-Paragraph 53
-URI http://asc.fasb.org/extlink&oid=77888399&loc=SL77918982-209971

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Balance Type:	debit
Period Type:	duration

- Definition

Amount of amortization expense attributable to right-of-use asset from finance lease.

+ References

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- Definition

Amount of lease cost recognized by lessee for lease contract.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918686-209980

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 55
-Paragraph 53
-URI http://asc.fasb.org/extlink&oid=77888399&loc=SL77918982-209971

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- References

No definition available.

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- Definition

Amount of single lease cost, calculated by allocation of remaining cost of lease over remaining lease term, amount of variable lease cost, excluded from lease liability, recognized when obligation for payment is incurred for finance and operating leases, and amount of short-term lease cost, excluding expense for lease with term of one month or less.

+ References

No definition available.

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- Definition

Amount of variable lease cost, excluded from lease liability, recognized when obligation for payment is incurred for finance and operating leases, and amount of short-term lease cost, excluding expense for lease with term of one month or less.

+ References

No definition available.

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