Notional Amounts for Each Category of Derivative Activity (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Derivative [Line Items]
Balance at beginning of period	$ 1,681.9	$ 1,620.7	$ 2,532.1
Additions	46.9	715.6	76.8
Terminations	315.8	654.4	988.2
Balance at ending of period	1,413.0	1,681.9	1,620.7
Swaps | Receive Variable/Pay Fixed
Derivative [Line Items]
Balance at beginning of period	174.0	174.0	174.0
Additions	0	250.0	0
Terminations	0	250.0	0
Balance at ending of period	174.0	174.0	174.0
Swaps | Receive Fixed/Pay Fixed
Derivative [Line Items]
Balance at beginning of period	617.9	661.9	931.8
Additions	0	0	70.9
Terminations	63.9	44.0	340.8
Balance at ending of period	554.0	617.9	661.9
Swaps | Receive Fixed/Pay Variable
Derivative [Line Items]
Balance at beginning of period	890.0	780.0	1,160.0
Additions	0	350.0	0
Terminations	205.0	240.0	380.0
Balance at ending of period	685.0	890.0	780.0
Forwards
Derivative [Line Items]
Balance at beginning of period	0	4.8	266.3
Additions	46.9	115.6	5.9
Terminations	46.9	120.4	267.4
Balance at ending of period	$ 0	$ 0	$ 4.8