Notional Amounts for Each Category of Derivative Activity (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Derivative [Line Items]
Balance at beginning of period	$ 1,432.8	$ 1,413.0	$ 1,681.9
Additions	291.0	336.0	46.9
Terminations	246.4	316.2	315.8
Balance at ending of period	1,477.4	1,432.8	1,413.0
Credit Default Swaps
Derivative [Line Items]
Balance at ending of period	97.0	0
Swaps | Receive Variable/Pay Fixed
Derivative [Line Items]
Balance at beginning of period	174.0	174.0	174.0
Additions	0	0	0
Terminations	24.0	0	0
Balance at ending of period	150.0	174.0	174.0
Swaps | Receive Fixed/Pay Fixed
Derivative [Line Items]
Balance at beginning of period	508.8	554.0	617.9
Additions	160.0	0	0
Terminations	38.4	45.2	63.9
Balance at ending of period	630.4	508.8	554.0
Swaps | Receive Fixed/Pay Variable
Derivative [Line Items]
Balance at beginning of period	750.0	685.0	890.0
Additions	0	250.0	0
Terminations	150.0	185.0	205.0
Balance at ending of period	600.0	750.0	685.0
Swaps | Credit Default Swaps
Derivative [Line Items]
Balance at beginning of period	0	0	0
Additions	97.0	0	0
Terminations	0	0	0
Balance at ending of period	97.0	0	0
Forwards
Derivative [Line Items]
Balance at beginning of period	0	0	0
Additions	24.0	86.0	46.9
Terminations	24.0	86.0	46.9
Balance at ending of period	0	0	0
Options Held
Derivative [Line Items]
Balance at beginning of period	0	0	0
Additions	10.0	0	0
Terminations	10.0	0	0
Balance at ending of period	$ 0	$ 0	$ 0