Stock Awards - Stock-based Award Activity (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Vesting Based on Service [Member]
Employee Restricted Equity Awards Vesting Activity [Line Items]
Nonvested awards at beginning of the year, Shares	923,848	276,280
Awarded, Shares	681,378	958,480
Vested, Shares	(478,104)	(307,275)
Forfeited, Shares	(4,682)	(3,637)
Nonvested awards at end of year, Shares	1,122,440	923,848
Nonvested awards at beginning of the year, Weighted Average Value at Award Date	$ 14.29	$ 12.68
Awarded, Weighted Average Value at Award Date	19.24	14.31
Vested, Weighted Average Value at Award Date	14.73	12.92
Forfeited, Weighted Average Value at Award Date	13.44	13.05
Nonvested awards at end of year, Weighted Average Value at Award Date	$ 17.11	$ 14.29
Vesting Based on Market/Performance Conditions [Member]
Employee Restricted Equity Awards Vesting Activity [Line Items]
Nonvested awards at beginning of the year, Shares	4,133,435	2,676,755
Awarded, Shares	2,438,292	1,750,834
Vested, Shares	(1,051,637)	(288,404)
Forfeited, Shares	(38,935)	(5,750)
Nonvested awards at end of year, Shares	5,481,155	4,133,435
Nonvested awards at beginning of the year, Weighted Average Value at Award Date	$ 9.21	$ 7.86
Awarded, Weighted Average Value at Award Date	15.25	11.61
Vested, Weighted Average Value at Award Date	10.43	11.25
Forfeited, Weighted Average Value at Award Date	10.13	9.35
Nonvested awards at end of year, Weighted Average Value at Award Date	$ 11.66	$ 9.21