Accumulated other comprehensive income (loss) - Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	$ (5.2)	$ 5.6	$ 10.7	$ (47.3)
Amounts Arising During the Current Period, Income Tax	0.4	0.3	0.8	1.5
Amounts Reclassified to Net Income (Loss), Pretax	13.4	14.7	26.5	28.4
Amounts Reclassified to Net Income (Loss), Income Tax	(4.6)	(4.8)	(8.8)	(10.0)
Other comprehensive income (loss)	4.0	15.8	29.2	(27.4)
Benefit plan adjustments, Attributable to parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(0.8)	(1.1)	(1.9)	(5.7)
Amounts Arising During the Current Period, Income Tax	0.2	0.3	0.5	1.5
Amounts Reclassified to Net Income (Loss), Pretax	12.6	14.1	24.9	29.8
Amounts Reclassified to Net Income (Loss), Income Tax	(4.5)	(4.8)	(8.7)	(10.0)
Other comprehensive income (loss)	7.5	8.5	14.8	15.6
Foreign currency translation adjustments, Attributable to parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(3.5)	6.5	13.6	(42.4)
Amounts Arising During the Current Period, Income Tax	0.0	0.0	0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.0	0.0	0.0	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0	0.0	0.0
Other comprehensive income (loss)	(3.5)	6.5	13.6	(42.4)
Unrealized gains (losses) on available-for-sale securities, Attributable to parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	0.3	0.0	0.5	0.0
Amounts Arising During the Current Period, Income Tax	(0.2)	0.0	(0.2)	0.0
Amounts Reclassified to Net Income (Loss), Pretax	(0.5)	(0.1)	(0.5)	(0.1)
Amounts Reclassified to Net Income (Loss), Income Tax	0.2	0.0	0.2	0.0
Other comprehensive income (loss)	(0.2)	(0.1)	0.0	(0.1)
Gains (losses) on cash flow hedges, Attributable to parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(1.2)	(0.2)	(2.2)	1.9
Amounts Arising During the Current Period, Income Tax	0.4	0.0	0.5	0.0
Amounts Reclassified to Net Income (Loss), Pretax	1.1	0.6	1.8	(1.5)
Amounts Reclassified to Net Income (Loss), Income Tax	(0.3)	0.0	(0.3)	0.0
Other comprehensive income (loss)	0.0	0.4	(0.2)	0.4
Accumulated Other Comprehensive Loss
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(5.2)	5.2	10.0	(46.2)
Amounts Arising During the Current Period, Income Tax	0.4	0.3	0.8	1.5
Amounts Reclassified to Net Income (Loss), Pretax	13.2	14.6	26.2	28.2
Amounts Reclassified to Net Income (Loss), Income Tax	(4.6)	(4.8)	(8.8)	(10.0)
Other comprehensive income (loss)	3.8	15.3	28.2	(26.5)
Benefit plan adjustments, Attributable to noncontrolling interest
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	0.0	0.0	0.0	0.0
Amounts Arising During the Current Period, Income Tax	0.0	0.0	0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.2	0.1	0.3	0.2
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0	0.0	0.0
Other comprehensive income (loss)	0.2	0.1	0.3	0.2
Foreign currency translation adjustments, Attributable to noncontrolling interest
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	0.0	0.4	0.7	(1.1)
Amounts Arising During the Current Period, Income Tax	0.0	0.0	0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.0	0.0	0.0	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0	0.0	0.0
Other comprehensive income (loss)	0.0	0.4	0.7	(1.1)
AOCI Attributable to Noncontrolling Interest [Member]
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	0.0	0.4	0.7	(1.1)
Amounts Arising During the Current Period, Income Tax	0.0	0.0	0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.2	0.1	0.3	0.2
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0	0.0	0.0
Other comprehensive income (loss)	0.2	0.5	1.0	(0.9)
Benefit plan adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(0.8)	(1.1)	(1.9)	(5.7)
Amounts Arising During the Current Period, Income Tax	0.2	0.3	0.5	1.5
Amounts Reclassified to Net Income (Loss), Pretax	12.8	14.2	25.2	30.0
Amounts Reclassified to Net Income (Loss), Income Tax	(4.5)	(4.8)	(8.7)	(10.0)
Other comprehensive income (loss)	7.7	8.6	15.1	15.8
Foreign currency translation adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(3.5)	6.9	14.3	(43.5)
Amounts Arising During the Current Period, Income Tax	0.0	0.0	0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.0	0.0	0.0	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0	0.0	0.0
Other comprehensive income (loss)	(3.5)	6.9	14.3	(43.5)
Unrealized gains (losses) on available-for-sale securities
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	0.3	0.0	0.5	0.0
Amounts Arising During the Current Period, Income Tax	(0.2)	0.0	(0.2)	0.0
Amounts Reclassified to Net Income (Loss), Pretax	(0.5)	(0.1)	(0.5)	(0.1)
Amounts Reclassified to Net Income (Loss), Income Tax	0.2	0.0	0.2	0.0
Other comprehensive income (loss)	(0.2)	(0.1)	0.0	(0.1)
Gains (losses) on cash flow hedges
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(1.2)	(0.2)	(2.2)	1.9
Amounts Arising During the Current Period, Income Tax	0.4	0.0	0.5	0.0
Amounts Reclassified to Net Income (Loss), Pretax	1.1	0.6	1.8	(1.5)
Amounts Reclassified to Net Income (Loss), Income Tax	(0.3)	0.0	(0.3)	0.0
Other comprehensive income (loss)	$ 0.0	$ 0.4	$ (0.2)	$ 0.4