Capital Stock (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class Of Stock [Line Items]
Beginning balance	247,144,356	169,316,460	168,628,177
Stock options/SARs exercised	0	0	77,002
Restricted stock grants	539,096	490,609	335,103
Shares retired	0	(739)	0
Treasury shares	15,580	28,736	23,671
Ending balance	248,129,430	247,144,356	169,316,460
MRD
Class Of Stock [Line Items]
MRD Merger	0	77,042,749	0
Restricted Stock
Class Of Stock [Line Items]
Restricted stock units vested	344,937	266,541	252,507
Performance Share
Class Of Stock [Line Items]
Performance stock units issued	85,461	0	0