Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity and Redeemable Noncontrolling Interest (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Schedule Of Stockholders Equity [Line Items]
Balance			$ 1,592,325
Balance			$ 16,108
Balance, shares			47,377,575
Common stock issuances			$ 5,623
Net income (loss)			86,681
Net income (loss)	$ 2,907	$ (4,210)	87,361	$ 72,732	$ 154,059	$ 131,629
Redemption value adjustments			(1,009)
Foreign currency exchange translation adj.			593
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	607	708	1,819	2,124	(583)	(9,324)
Amounts reclassified into net income	518	$ 519	1,556	$ 1,556	2,073	$ 2,074
Dividends declared Common			(64,672)
Balance	1,622,916		1,622,916		1,622,916
Balance	$ 17,379		$ 17,379		$ 17,379
Balance, shares	47,482,068		47,482,068		47,482,068
Redeemable Noncontrolling Interest (Temporary Equity) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			$ 16,108
Net income (loss) attributable to temporary equity			680
Redemption value adjustments			1,009
Foreign currency exchange translation adj attributable to temporary equity			21
Other comprehensive income (loss):
Centuri dividend to redeemable noncontrolling interest			(439)
Centuri dividend to redeemable noncontrolling interest			(439)
Balance	$ 17,379		17,379		$ 17,379
Non-controlling Interest [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			(2,083)
Net income (loss)			(180)
Other comprehensive income (loss):
Balance	(2,263)		(2,263)		(2,263)
Common Stock [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			$ 49,007
Balance, shares			47,377,000
Common stock issuances			$ 105
Common stock issuances, Shares			105,000
Other comprehensive income (loss):
Balance	$ 49,112		$ 49,112		$ 49,112
Balance, shares	47,482,000		47,482,000		47,482,000
Additional Paid-in Capital [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			$ 896,448
Common stock issuances			5,518
Other comprehensive income (loss):
Balance	$ 901,966		901,966		$ 901,966
Accumulated Other Comprehensive Income (Loss) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			(50,268)
Foreign currency exchange translation adj.			593
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax			1,819
Amounts reclassified into net income			1,556
Balance	(46,300)		(46,300)		(46,300)
Retained Earnings [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			699,221
Net income (loss)			86,861
Redemption value adjustments			(1,009)
Other comprehensive income (loss):
Dividends declared Common			(64,672)
Balance	$ 720,401		$ 720,401		$ 720,401