Notional Amounts for Each Category of Derivative Activity (Detail) (USD $) In Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2011
Derivative [Line Items]
Balance at beginning of period	$ 1,615.9	$ 1,681.9
Additions	12.0	31.9
Terminations	89.2	175.1
Balance at ending of period	1,538.7	1,538.7
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	606.9	617.9
Additions	0	0
Terminations	27.2	38.2
Balance at ending of period	579.7	579.7
Swaps | Receive Fixed/Pay Variable
Derivative [Line Items]
Balance at beginning of period	835.0	890.0
Additions	0	0
Terminations	50.0	105.0
Balance at ending of period	785.0	785.0
Forwards
Derivative [Line Items]
Balance at beginning of period	0	0
Additions	12.0	31.9
Terminations	12.0	31.9
Balance at ending of period	0	0
Receive Fixed/Pay Variable
Derivative [Line Items]
Balance at ending of period	$ 435.0	$ 435.0