Retirement plans - Narrative (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
401(k) employee maximum matching contribution		50.00%
401(k) employer matching contribution		100.00%
Contributions per employee subject to employer match		4.00%
Compensation expense related to 401(k) contributions		$ 16,500,000	$ 14,600,000	$ 13,300,000
Expenses incurred for multi-employer withdrawal liabilities		0	0	$ 0
Retirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarial losses		$ 48,491,000	$ 73,906,000
Discount rate (as a percent)	3.29%	2.54%	3.29%	4.34%
Actuarial losses from change in discount rate		$ 49,300,000	$ 61,400,000
Actuarial losses from change in mortality assumption			11,300,000
Accumulated benefit obligation	$ 613,655,000	646,644,000	613,655,000
Discretionary contributions by employer	$ 12,000,000
Contributions made to SERP		5,133,000	$ 23,100,000
Contributions expected to be made during next fiscal year		$ 7,700,000
Actual rate of return on plan assets (as a percent)		23.50%	23.60%	(5.60%)
SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contributions made to SERP		$ 5,000,000.0
AFTRA Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contribution by employer		$ 2,400,000	$ 2,400,000	$ 2,400,000