Income Taxes - Roll-Forward of Unrecognized Tax Benefits and Associated Accrued Interest and Penalties (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Income Tax Contingency [Line Items]
Opening balance at January 1, 2015	$ 6.2
Additions for tax positions of prior periods	0.1
Closing balance at June 30, 2015	6.3
Current	0.0
Non-current	6.3	$ 6.2
Unrecognized Tax Benefits [Member]
Income Tax Contingency [Line Items]
Opening balance at January 1, 2015	5.7
Additions for tax positions of prior periods	0.0
Closing balance at June 30, 2015	5.7
Current	0.0
Non-current	5.7
Interest and Penalties [Member]
Income Tax Contingency [Line Items]
Opening balance at January 1, 2015	0.5
Additions for tax positions of prior periods	0.1
Closing balance at June 30, 2015	0.6
Current	0.0
Non-current	$ 0.6