Notional Amounts for Each Category of Derivative Activity (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Derivative [Line Items]
Balance at beginning of period	$ 1,309,500,000	$ 1,538,700,000	$ 1,413,000,000	$ 1,681,900,000
Additions	275,000,000	15,000,000	331,000,000	46,900,000
Terminations	71,300,000	55,000,000	230,800,000	230,100,000
Balance at ending of period	1,513,200,000	1,498,700,000	1,513,200,000	1,498,700,000
Swaps | Receive Variable/Pay Fixed
Derivative [Line Items]
Balance at beginning of period	174,000,000	174,000,000	174,000,000	174,000,000
Additions	0	0	0	0
Terminations	0	0	0	0
Balance at ending of period	174,000,000	174,000,000	174,000,000	174,000,000
Swaps | Receive Fixed/Pay Fixed
Derivative [Line Items]
Balance at beginning of period	540,500,000	579,700,000	554,000,000	617,900,000
Additions	0	0	0	0
Terminations	26,300,000	5,000,000	39,800,000	43,200,000
Balance at ending of period	514,200,000	574,700,000	514,200,000	574,700,000
Swaps | Receive Fixed/Pay Variable
Derivative [Line Items]
Balance at beginning of period	595,000,000	785,000,000	685,000,000	890,000,000
Additions	250,000,000	0	250,000,000	0
Terminations	45,000,000	50,000,000	135,000,000	155,000,000
Balance at ending of period	800,000,000	735,000,000	800,000,000	735,000,000
Forwards
Derivative [Line Items]
Balance at beginning of period	0	0	0	0
Additions	25,000,000	15,000,000	81,000,000	46,900,000
Terminations	0	0	56,000,000	31,900,000
Balance at ending of period	$ 25,000,000	$ 15,000,000	$ 25,000,000	$ 15,000,000