XML 155 R121.htm IDEA: XBRL DOCUMENT v3.8.0.1
Stock-Based Compensation - Summary of Performance Share Units Plan (Detail) - Performance Share Units [Member] - shares
3 Months Ended 12 Months Ended
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward] (in shares)      
Outstanding, January 1 373,593 373,593 348,276
Granted   134,991 147,157
Units, in lieu of dividends   3,571 4,010
Settled (133,728) (133,728) (83,466)
Forfeited   (44,399) (42,384)
Outstanding, December 31   334,028 373,593