Notional Amounts for Each Category of Derivative Activity (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Derivative [Line Items]
Balance at beginning of period	$ 1,364.1	$ 1,615.9	$ 1,413.0	$ 1,681.9
Additions	21.0	12.0	56.0	31.9
Terminations	75.6	89.2	159.5	175.1
Balance at ending of period	1,309.5	1,538.7	1,309.5	1,538.7
Swaps | Receive Variable/Pay Fixed
Derivative [Line Items]
Balance at beginning of period	174.0	174.0	174.0	174.0
Additions	0	0	0	0
Terminations	0	0	0	0
Balance at ending of period	174.0	174.0	174.0	174.0
Swaps | Receive Fixed/Pay Fixed
Derivative [Line Items]
Balance at beginning of period	550.1	606.9	554.0	617.9
Additions	0	0	0	0
Terminations	9.6	27.2	13.5	38.2
Balance at ending of period	540.5	579.7	540.5	579.7
Swaps | Receive Fixed/Pay Variable
Derivative [Line Items]
Balance at beginning of period	640.0	835.0	685.0	890.0
Additions	0	0	0	0
Terminations	45.0	50.0	90.0	105.0
Balance at ending of period	595.0	785.0	595.0	785.0
Forwards
Derivative [Line Items]
Balance at beginning of period	0	0	0	0
Additions	21.0	12.0	56.0	31.9
Terminations	21.0	12.0	56.0	31.9
Balance at ending of period	$ 0	$ 0	$ 0	$ 0