Notional Amounts for Each Category of Derivative Activity (Detail) (USD $) In Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2011
Derivative [Line Items]
Balance at beginning of period	$ 1,538.7	$ 1,681.9
Additions	15.0	46.9
Terminations	55.0	230.1
Balance at ending of period	1,498.7	1,498.7
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	579.7	617.9
Additions	0	0
Terminations	5.0	43.2
Balance at ending of period	574.7	574.7
Swaps | Receive Fixed/Pay Variable
Derivative [Line Items]
Balance at beginning of period	785.0	890.0
Additions	0	0
Terminations	50.0	155.0
Balance at ending of period	735.0	735.0
Forwards
Derivative [Line Items]
Balance at beginning of period	0	0
Additions	15.0	46.9
Terminations	0	31.9
Balance at ending of period	15.0	15.0
Receive Fixed/Pay Variable
Derivative [Line Items]
Balance at ending of period	$ 385.0	$ 385.0