Notional Amounts for Each Category of Derivative Activity (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended				3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 Credit Default Swaps	Dec. 31, 2012 Credit Default Swaps	Sep. 30, 2013 Swaps Receive Variable/Pay Fixed	Sep. 30, 2012 Swaps Receive Variable/Pay Fixed	Sep. 30, 2013 Swaps Receive Variable/Pay Fixed	Sep. 30, 2012 Swaps Receive Variable/Pay Fixed	Sep. 30, 2013 Swaps Receive Fixed/Pay Fixed	Sep. 30, 2012 Swaps Receive Fixed/Pay Fixed	Sep. 30, 2013 Swaps Receive Fixed/Pay Fixed	Sep. 30, 2012 Swaps Receive Fixed/Pay Fixed	Sep. 30, 2013 Swaps Receive Fixed/Pay Variable	Sep. 30, 2012 Swaps Receive Fixed/Pay Variable	Sep. 30, 2013 Swaps Receive Fixed/Pay Variable	Sep. 30, 2012 Swaps Receive Fixed/Pay Variable	Sep. 30, 2013 Swaps Credit Default Swaps	Sep. 30, 2012 Swaps Credit Default Swaps	Sep. 30, 2013 Swaps Credit Default Swaps	Sep. 30, 2012 Swaps Credit Default Swaps	Sep. 30, 2013 Forwards	Sep. 30, 2012 Forwards	Sep. 30, 2013 Forwards	Sep. 30, 2012 Forwards	Sep. 30, 2013 Options Held	Sep. 30, 2012 Options Held	Sep. 30, 2013 Options Held	Sep. 30, 2012 Options Held
Derivative [Line Items]
Balance at beginning of period	$ 1,403.7	$ 1,309.5	$ 1,432.8	$ 1,413.0	$ 70.0	$ 0	$ 174.0	$ 174.0	$ 174.0	$ 174.0	$ 474.7	$ 540.5	$ 508.8	$ 554.0	$ 675.0	$ 595.0	$ 750.0	$ 685.0	$ 70.0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 10.0	$ 0	$ 0	$ 0
Additions	10.0	275.0	114.0	331.0			0	0	0	0	10.0	0	10.0	0	0	250.0	0	250.0	0	0	70.0	0	0	25.0	24.0	81.0	0	0	10.0	0
Terminations	77.6	71.3	210.7	230.8			24.0	0	24.0	0	3.6	26.3	37.7	39.8	40.0	45.0	115.0	135.0	0	0	0	0	0	0	24.0	56.0	10.0	0	10.0	0
Balance at ending of period	$ 1,336.1	$ 1,513.2	$ 1,336.1	$ 1,513.2	$ 70.0	$ 0	$ 150.0	$ 174.0	$ 150.0	$ 174.0	$ 481.1	$ 514.2	$ 481.1	$ 514.2	$ 635.0	$ 800.0	$ 635.0	$ 800.0	$ 70.0	$ 0	$ 70.0	$ 0	$ 0	$ 25.0	$ 0	$ 25.0	$ 0	$ 0	$ 0	$ 0