Business and Significant Accounting Policies - Effects of Retrospective Adoption on Individual Financial Statement Line Items in Consolidated Statements of Comprehensive (Loss) Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Basis Of Presentation [Line Items]
Net income (loss)	$ (4.7)	$ 13.4	$ 24.0	$ 19.6	$ 1.4	$ (10.8)	$ 21.6	$ (94.1)	$ 52.3	$ (81.9)	$ 86.7
Comprehensive income (loss)	(19.3)	60.0	6.2	55.1	27.1	(61.9)	31.3	(86.2)	102.0	(89.7)	126.9
As Previously Reported [Member]
Basis Of Presentation [Line Items]
Net income (loss)										(82.4)	82.6
Comprehensive income (loss)										(90.2)	122.8
As Adjusted [Member]
Basis Of Presentation [Line Items]
Net income (loss)										(81.9)	86.7
Comprehensive income (loss)										(89.7)	126.9
Effect of Change [Member]
Basis Of Presentation [Line Items]
Net income (loss)										0.5	4.1
Comprehensive income (loss)										$ 0.5	$ 4.1