Capital Stock (Detail) - shares	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Class Of Stock [Line Items]
Beginning balance	247,144,356	169,316,460
Restricted stock grants	536,536	490,609
Restricted stock units vested	341,358	266,541
PSU-TSR units settled	85,461	0
Shares retired	0	(739)
Treasury shares issued	15,580	28,736
Ending balance	248,123,291	247,144,356
MRD
Class Of Stock [Line Items]
MRD Merger	0	77,042,749