Notional Amounts for Each Category of Derivative Activity (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014 Credit Default Swaps	Dec. 31, 2013 Credit Default Swaps	Mar. 31, 2014 Swaps Receive Variable/Pay Fixed	Mar. 31, 2013 Swaps Receive Variable/Pay Fixed	Mar. 31, 2014 Swaps Receive Fixed/Pay Fixed	Mar. 31, 2013 Swaps Receive Fixed/Pay Fixed	Mar. 31, 2014 Swaps Receive Fixed/Pay Variable	Mar. 31, 2013 Swaps Receive Fixed/Pay Variable	Mar. 31, 2014 Swaps Credit Default Swaps	Mar. 31, 2013 Swaps Credit Default Swaps	Mar. 31, 2014 Forwards	Mar. 31, 2013 Forwards
Derivative
Balance at beginning of period	$ 1,477.4	$ 1,432.8	$ 97.0	$ 97.0	$ 150.0	$ 174.0	$ 630.4	$ 508.8	$ 600.0	$ 750.0	$ 97.0	$ 0	$ 0	$ 0
Additions	10.0	94.0			0	0	0	0	0	0	0	70.0	10.0	24.0
Terminations	17.9	82.0			0	0	7.9	18.0	0	40.0	0	0	10.0	24.0
Balance at ending of period	$ 1,469.5	$ 1,444.8	$ 97.0	$ 97.0	$ 150.0	$ 174.0	$ 622.5	$ 490.8	$ 600.0	$ 710.0	$ 97.0	$ 70.0	$ 0	$ 0