Significant Accounting Policies - Changes in Accumulated Other Comprehensive Loss Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 4,975,496	$ 4,679,830	$ 4,949,412	$ 4,682,477
Other comprehensive earnings (loss) before reclassifications, net of tax	264	(476)	761	(1,063)
Amounts reclassified from accumulated other comprehensive loss, net of tax	2,726	2,078	5,484	4,284
Other comprehensive earnings (loss), net of tax	2,990	1,602	6,245	3,221
Ending Balance	5,097,270	4,855,470	5,097,270	4,855,470
Pension and Postretirement Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(138,747)	(126,806)	(141,505)	(128,802)
Amounts reclassified from accumulated other comprehensive loss, net of tax	2,726	2,008	5,484	4,004
Other comprehensive earnings (loss), net of tax	2,726	2,008	5,484	4,004
Ending Balance	(136,021)	(124,798)	(136,021)	(124,798)
Foreign Currency
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(1,577)	(609)	(2,074)	(22)
Other comprehensive earnings (loss) before reclassifications, net of tax	264	476	761	(1,063)
Other comprehensive earnings (loss), net of tax	264	476	761	(1,063)
Ending Balance	(1,313)	(1,085)	(1,313)	(1,085)
Unamortized Value of Terminated Forward Starting Interest Rate Swap
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(70)		(280)
Amounts reclassified from accumulated other comprehensive loss, net of tax		70		280
Other comprehensive earnings (loss), net of tax		70		280
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(140,324)	(127,485)	(143,579)	(129,104)
Ending Balance	$ (137,334)	$ (125,883)	$ (137,334)	$ (125,883)