Stock Awards - Stock-based Award Activity (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Vesting Based on Service [Member]
Employee Restricted Equity Awards Vesting Activity [Line Items]
Nonvested awards at beginning of the year, Shares	1,122,440	923,848
Awarded, Shares	635,855	681,378
Vested, Shares	(699,215)	(478,104)
Forfeited, Shares	(2,026)	(4,682)
Nonvested awards at end of year, Shares	1,057,054	1,122,440
Nonvested awards at beginning of the year, Weighted Average Value at Award Date	$ 17.11	$ 14.29
Awarded, Weighted Average Value at Award Date	19.65	19.24
Vested, Weighted Average Value at Award Date	16.80	14.73
Forfeited, Weighted Average Value at Award Date	18.40	13.44
Nonvested awards at end of year, Weighted Average Value at Award Date	$ 18.79	$ 17.11
Vesting Based on Market/Performance Conditions [Member]
Employee Restricted Equity Awards Vesting Activity [Line Items]
Nonvested awards at beginning of the year, Shares	5,481,155	4,133,435
Awarded, Shares	1,800,898	2,438,292
Vested, Shares	(2,193,906)	(1,051,637)
Forfeited, Shares	(1,164)	(38,935)
Nonvested awards at end of year, Shares	5,086,983	5,481,155
Nonvested awards at beginning of the year, Weighted Average Value at Award Date	$ 11.66	$ 9.21
Awarded, Weighted Average Value at Award Date	19.42	15.25
Vested, Weighted Average Value at Award Date	11.35	10.43
Forfeited, Weighted Average Value at Award Date	18.22	10.13
Nonvested awards at end of year, Weighted Average Value at Award Date	$ 14.41	$ 11.66