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, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 Credit Default Swaps	Dec. 31, 2013 Credit Default Swaps	Jun. 30, 2014 Swaps Receive Variable/Pay Fixed	Jun. 30, 2013 Swaps Receive Variable/Pay Fixed	Jun. 30, 2014 Swaps Receive Variable/Pay Fixed	Jun. 30, 2013 Swaps Receive Variable/Pay Fixed	Jun. 30, 2014 Swaps Receive Fixed/Pay Fixed	Jun. 30, 2013 Swaps Receive Fixed/Pay Fixed	Jun. 30, 2014 Swaps Receive Fixed/Pay Fixed	Jun. 30, 2013 Swaps Receive Fixed/Pay Fixed	Jun. 30, 2014 Swaps Receive Fixed/Pay Variable	Jun. 30, 2013 Swaps Receive Fixed/Pay Variable	Jun. 30, 2014 Swaps Receive Fixed/Pay Variable	Jun. 30, 2013 Swaps Receive Fixed/Pay Variable	Jun. 30, 2014 Swaps Credit Default Swaps	Jun. 30, 2013 Swaps Credit Default Swaps	Jun. 30, 2014 Swaps Credit Default Swaps	Jun. 30, 2013 Swaps Credit Default Swaps	Jun. 30, 2014 Forwards	Jun. 30, 2013 Forwards	Jun. 30, 2014 Forwards	Jun. 30, 2013 Forwards	Jun. 30, 2014 Options Held	Jun. 30, 2013 Options Held	Jun. 30, 2014 Options Held	Jun. 30, 2013 Options Held
Derivative
Balance at beginning of period	$ 1,469.5	$ 1,444.8	$ 1,477.4	$ 1,432.8	$ 97.0	$ 97.0	$ 150.0	$ 174.0	$ 150.0	$ 174.0	$ 622.5	$ 490.8	$ 630.4	$ 508.8	$ 600.0	$ 710.0	$ 600.0	$ 750.0	$ 97.0	$ 70.0	$ 97.0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Additions	0	10.0	10.0	104.0			0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	70.0	0	0	10.0	24.0	0	10.0	0	10.0
Terminations	14.2	51.1	32.1	133.1			0	0	0	0	14.2	16.1	22.1	34.1	0	35.0	0	75.0	0	0	0	0	0	0	10.0	24.0	0	0	0	0
Balance at ending of period	$ 1,455.3	$ 1,403.7	$ 1,455.3	$ 1,403.7	$ 97.0	$ 97.0	$ 150.0	$ 174.0	$ 150.0	$ 174.0	$ 608.3	$ 474.7	$ 608.3	$ 474.7	$ 600.0	$ 675.0	$ 600.0	$ 675.0	$ 97.0	$ 70.0	$ 97.0	$ 70.0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 10.0	$ 0	$ 10.0