Notional Amounts for Each Category of Derivative Activity (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Derivative [Line Items]
Balance at beginning of period	$ 1,432.8	$ 1,413.0
Additions	94.0	35.0
Terminations	82.0	83.9
Balance at ending of period	1,444.8	1,364.1
Credit Default Swap [Member]
Derivative [Line Items]
Additions	70.0
Swaps | Receive Variable/Pay Fixed
Derivative [Line Items]
Balance at beginning of period	174.0	174.0
Additions	0	0
Terminations	0	0
Balance at ending of period	174.0	174.0
Swaps | Receive Fixed/Pay Fixed
Derivative [Line Items]
Balance at beginning of period	508.8	554.0
Additions	0	0
Terminations	18.0	3.9
Balance at ending of period	490.8	550.1
Swaps | Receive Fixed/Pay Variable
Derivative [Line Items]
Balance at beginning of period	750.0	685.0
Additions	0	0
Terminations	40.0	45.0
Balance at ending of period	710.0	640.0
Swaps | Credit Default Swap [Member]
Derivative [Line Items]
Balance at beginning of period	0	0
Additions	70.0	0
Terminations	0	0
Balance at ending of period	70.0	0
Forwards
Derivative [Line Items]
Balance at beginning of period	0	0
Additions	24.0	35.0
Terminations	24.0	35.0
Balance at ending of period	$ 0	$ 0