Notional Amounts for Each Category of Derivative Activity (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended				3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Credit Default Swaps	Dec. 31, 2012 Credit Default Swaps	Jun. 30, 2013 Swaps Receive Variable/Pay Fixed	Jun. 30, 2012 Swaps Receive Variable/Pay Fixed	Jun. 30, 2013 Swaps Receive Variable/Pay Fixed	Jun. 30, 2012 Swaps Receive Variable/Pay Fixed	Jun. 30, 2013 Swaps Receive Fixed/Pay Fixed	Jun. 30, 2012 Swaps Receive Fixed/Pay Fixed	Jun. 30, 2013 Swaps Receive Fixed/Pay Fixed	Jun. 30, 2012 Swaps Receive Fixed/Pay Fixed	Jun. 30, 2013 Swaps Receive Fixed/Pay Variable	Jun. 30, 2012 Swaps Receive Fixed/Pay Variable	Jun. 30, 2013 Swaps Receive Fixed/Pay Variable	Jun. 30, 2012 Swaps Receive Fixed/Pay Variable	Jun. 30, 2013 Swaps Credit Default Swaps	Jun. 30, 2012 Swaps Credit Default Swaps	Jun. 30, 2013 Swaps Credit Default Swaps	Jun. 30, 2012 Swaps Credit Default Swaps	Jun. 30, 2013 Forwards	Jun. 30, 2012 Forwards	Jun. 30, 2013 Forwards	Jun. 30, 2012 Forwards	Jun. 30, 2013 Options Held	Jun. 30, 2012 Options Held	Jun. 30, 2013 Options Held	Jun. 30, 2012 Options Held
Derivative [Line Items]
Balance at beginning of period	$ 1,444.8	$ 1,364.1	$ 1,432.8	$ 1,413.0	$ 70.0	$ 0	$ 174.0	$ 174.0	$ 174.0	$ 174.0	$ 490.8	$ 550.1	$ 508.8	$ 554.0	$ 710.0	$ 640.0	$ 750.0	$ 685.0	$ 70.0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Additions	10.0	21.0	104.0	56.0			0	0	0	0	0	0	0	0	0	0	0	0	0	0	70.0	0	0	21.0	24.0	56.0	10.0	0	10.0	0
Terminations	51.1	75.6	133.1	159.5			0	0	0	0	16.1	9.6	34.1	13.5	35.0	45.0	75.0	90.0	0	0	0	0	0	21.0	24.0	56.0	0	0	0	0
Balance at ending of period	$ 1,403.7	$ 1,309.5	$ 1,403.7	$ 1,309.5	$ 70.0	$ 0	$ 174.0	$ 174.0	$ 174.0	$ 174.0	$ 474.7	$ 540.5	$ 474.7	$ 540.5	$ 675.0	$ 595.0	$ 675.0	$ 595.0	$ 70.0	$ 0	$ 70.0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 10.0	$ 0	$ 10.0	$ 0