Summary of Status of Senior Executive Plans (Parenthetical) (Detail) - Executive Officer - Restricted Stock - USD ($) shares in Millions, $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued	[1]	0.4	0.3	0.3
Value of shares withheld for employee tax requirements		$ 5.4	$ 4.8	$ 4.9

[1] Shares that vest at December 31 of each year are issued to the employees in the first quarter of the following year, although the requisite service period is complete. Accordingly, the 0.4 million shares issued in 2019 include shares vested at December 31, 2018, after adjusting for shares withheld to meet employee tax requirements. The shares withheld for employee tax requirements were valued at $5.4 million, $4.8 million and $4.9 million for 2019, 2018 and 2017, respectively.