Shareholders' Equity - Summary of Preferred and Common Share Activity (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in Stockholders' Equity
Common stock issued, beginning balance	127,218,183
Treasury stock, beginning balance	(9,187,075)
Grants of restricted stock	169,870	186,961	193,788
Vesting of performance shares	153,360	306,802	287,390
Issuance of common stock due to exercise of stock options	1,661,808	2,184,169	1,576,485
Retirement of treasury stock	3,000,000	3,000,000	4,000,000
Common stock issued, ending balance	124,218,183	127,218,183
Treasury stock, ending balance	(9,625,104)	(9,187,075)
Preferred Stock
Increase (Decrease) in Stockholders' Equity
Preferred stock issued, beginning balance	0	0	0
Treasury stock, beginning balance	0	0	0
Preferred stock issued, ending balance	0	0	0
Treasury stock, ending balance	0	0	0
Common Stock
Increase (Decrease) in Stockholders' Equity
Common stock issued, beginning balance	127,218,183	130,218,183.000	134,218,183
Treasury stock, beginning balance	(9,187,075)	(7,848,231.000)	(6,287,907)
Grants of restricted stock	9,135	12,549	6,648
Forfeitures of restricted stock		0	(13,950)
Issuance of common stock due to exercise of stock options	1,661,808	2,184,169	1,576,485
Treasury stock acquired	(5,228,868)	(6,694,582)	(7,340,794)
Retirement of treasury stock	3,000,000	3,000,000	4,000,000
Common stock issued, ending balance	124,218,183.000	127,218,183	130,218,183.000
Treasury stock, ending balance	(9,625,104.000)	(9,187,075)	(7,848,231.000)
Performance Shares
Increase (Decrease) in Stockholders' Equity
Grants of restricted stock	153,000	167,500	179,500
Performance Shares | Common Stock
Increase (Decrease) in Stockholders' Equity
Vesting of performance shares	119,896	159,020	211,287