Changes in Accumulated Other Comprehensive Earnings (Loss), Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 4,682,477	$ 4,142,590
Other comprehensive earnings (loss) before reclassifications, net of tax	(587)	137
Amounts reclassified from accumulated other comprehensive earnings, net of tax	2,206	2,125
Other comprehensive earnings (loss), net of tax	1,619	2,262
Ending Balance	4,679,830	4,075,954
Pension and Postretirement Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(128,802)	(128,373)
Amounts reclassified from accumulated other comprehensive earnings, net of tax	1,996	1,910
Other comprehensive earnings (loss), net of tax	1,996	1,910
Ending Balance	(126,806)	(126,463)
Foreign Currency
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(22)	(1,162)
Other comprehensive earnings (loss) before reclassifications, net of tax	(587)	137
Other comprehensive earnings (loss), net of tax	(587)	137
Ending Balance	(609)	(1,025)
Unamortized Value of Terminated Forward Starting Interest Rate Swap
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(280)	(1,152)
Amounts reclassified from accumulated other comprehensive earnings, net of tax	210	215
Other comprehensive earnings (loss), net of tax	210	215
Ending Balance	(70)	(937)
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(129,104)	(130,687)
Ending Balance	$ (127,485)	$ (128,425)