Significant Accounting Policies - Changes in Accumulated Other Comprehensive Earnings Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			$ 4,682,477	$ 4,142,590
Other comprehensive earnings before reclassifications, net of tax	$ 365	$ 838	(698)	1,364
Amounts reclassified from accumulated other comprehensive loss, net of tax	4,027	2,140	8,311	6,395
Other comprehensive earnings, net of tax	4,392	2,978	7,613	7,759
Ending Balance	4,942,328	4,333,982	4,942,328	4,333,982
Pension and Postretirement Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(124,798)	(124,553)	(128,802)	(128,373)
Amounts reclassified from accumulated other comprehensive loss, net of tax	4,027	1,918	8,031	5,738
Other comprehensive earnings, net of tax	4,027	1,918	8,031	5,738
Ending Balance	(120,771)	(122,635)	(120,771)	(122,635)
Foreign Currency
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(1,085)	(636)	(22)	(1,162)
Other comprehensive earnings before reclassifications, net of tax	365	838	(698)	1,364
Other comprehensive earnings, net of tax	365	838	(698)	1,364
Ending Balance	(720)	202	(720)	202
Unamortized Value of Terminated Forward Starting Interest Rate Swap
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(717)	(280)	(1,152)
Amounts reclassified from accumulated other comprehensive loss, net of tax		222	280	657
Other comprehensive earnings, net of tax		222	280	657
Ending Balance		(495)		(495)
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(125,883)	(125,906)	(129,104)	(130,687)
Ending Balance	$ (121,491)	$ (122,928)	$ (121,491)	$ (122,928)