Significant Accounting Policies - Changes in Accumulated Other Comprehensive Earnings (Loss), Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 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	$ (476)	$ 389	(1,063)	526
Amounts reclassified from accumulated other comprehensive earnings, net of tax	2,078	2,130	4,284	4,255
Other comprehensive earnings (loss), net of tax	1,602	2,519	3,221	4,781
Ending Balance	4,855,470	4,200,531	4,855,470	4,200,531
Pension and Postretirement Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(126,806)	(126,463)	(128,802)	(128,373)
Amounts reclassified from accumulated other comprehensive earnings, net of tax	2,008	1,910	4,004	3,820
Other comprehensive earnings (loss), net of tax	2,008	1,910	4,004	3,820
Ending Balance	(124,798)	(124,553)	(124,798)	(124,553)
Foreign Currency
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(609)	(1,025)	(22)	(1,162)
Other comprehensive earnings (loss) before reclassifications, net of tax	(476)	389	(1,063)	526
Other comprehensive earnings (loss), net of tax	(476)	389	(1,063)	526
Ending Balance	(1,085)	(636)	(1,085)	(636)
Unamortized Value of Terminated Forward Starting Interest Rate Swap
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(70)	(937)	(280)	(1,152)
Amounts reclassified from accumulated other comprehensive earnings, net of tax	70	220	280	435
Other comprehensive earnings (loss), net of tax	70	220	280	435
Ending Balance		(717)		(717)
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(127,485)	(128,425)	(129,104)	(130,687)
Ending Balance	$ (125,883)	$ (125,906)	$ (125,883)	$ (125,906)