Restricted Equity Awards (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Vesting Based on Service
Employee Restricted Equity Awards Vesting Activity [Line Items]
Nonvested awards at beginning of the year, Shares	603,980	783,305
Awarded, Shares	275,464	292,803
Vested, Shares	(410,261)	(453,980)
Forfeited, Shares	(2,300)	(18,148)
Nonvested awards at end of year, Shares	466,883	603,980
Nonvested awards at beginning of the year, Weighted Average Value at Award Date	$ 11.02	$ 10.43
Awarded, Weighted Average Value at Award Date	$ 10.14	$ 10.76
Vested, Weighted Average Value at Award Date	$ 10.78	$ 9.89
Forfeited, Weighted Average Value at Award Date	$ 10.24	$ 9.70
Nonvested awards at end of year, Weighted Average Value at Award Date	$ 10.72	$ 11.02
Vesting Based on Market/Performance Conditions
Employee Restricted Equity Awards Vesting Activity [Line Items]
Nonvested awards at beginning of the year, Shares	1,511,397	828,409
Awarded, Shares	902,359	853,656
Vested, Shares	(513,693)	(163,575)
Forfeited, Shares	(20,174)	(7,093)
Nonvested awards at end of year, Shares	1,879,889	1,511,397
Nonvested awards at beginning of the year, Weighted Average Value at Award Date	$ 7.21	$ 8.70
Awarded, Weighted Average Value at Award Date	$ 5.16	$ 5.72
Vested, Weighted Average Value at Award Date	$ 8.62	$ 6.87
Forfeited, Weighted Average Value at Award Date	$ 4.59	$ 9.32
Nonvested awards at end of year, Weighted Average Value at Award Date	$ 5.87	$ 7.21