Stock-based Compensation Plans - PSUA Activities (Parenthetical) (Detail) - shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Oklahoma, Virginia And West Virginia
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares vested in period on performance number of units	19,684
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Actual payout of shares on performance units granted	0.00%	0.00%	0.00%
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Actual payout of shares on performance units granted	150.00%	150.00%	150.00%