Quarterly financial data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended								12 Months Ended
		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, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly financial data (unaudited) [Abstract]
Revenue		$ 270.7	$ 173.7	$ 220.2	$ 117.8	$ 193.8	$ 149.4	$ 100.8	$ 116.9	$ 782.4	$ 560.9	$ 488.8
Income (loss) from operations		11.2	21.3	66.8	(9.5)	53.1	47.8	8.5	14.9
Net income from continuing operations		$ (3.4)	$ 20.9	$ 50.1	$ (4.9)	$ 33.9	$ 33.6	$ 4.6	$ 10.5	62.7	82.6	62.5
Net loss from discontinued operations										0.0	0.0	(10.7)
Net income (loss)										$ 62.7	$ 82.6	$ 51.8
Net income (loss) per share from continuing operations-basic (in dollars per share)		$ (0.07)	$ 0.42	$ 1.00	$ (0.10)	$ 0.77	$ 0.81	$ 0.11	$ 0.26	$ 1.25	$ 1.98	$ 1.56
Net loss per share from discontinued operations-basic (in dollars per share)										0	0	(0.27)
Net income per share-basic (in dollars per share)										1.25	1.98	1.29
Net income per share from continuing operations-diluted (in dollars per share)		$ (0.07)	$ 0.41	$ 0.98	$ (0.10)	$ 0.67	$ 0.68	$ 0.11	$ 0.23	1.22	1.71	1.35
Net income (loss) per share from discontinued operations-diluted (in dollars per share)										0	0	(0.22)
Net income per share-diluted (in dollars per share)	[1]									$ 1.22	$ 1.71	$ 1.13

[1] See "Earnings per share" footnote for details on calculation.