Retirement plans - Reconciliation of Benefit Obligations, Plan Assets and Funded Status of Company-Sponsored Retirement Plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in benefit obligations
Settlements (2)	$ (30,274)	$ 0
Benefit obligations at end of year	554,795
Change in plan assets
Acquisition of KFMB	18,694	0
Settlements (2)	(30,274)	0
Fair value of plan assets at end of year	407,550
Funded status at end of year	(147,245)
Amounts recognized in Consolidated Balance Sheets
Accrued benefit cost—noncurrent	(139,375)	(144,220)
Retirement Plans
Change in benefit obligations
Benefit obligations at beginning of year	614,111	606,413
Service cost	12	872	$ 816
Interest cost	21,337	23,985	26,111
Actuarial (gain) loss	(40,135)	26,700
Benefits paid	(36,222)	(39,143)
Acquisition of KFMB	25,966	0
Curtailment gain	0	(4,716)
Benefit obligations at end of year	554,795	614,111	606,413
Change in plan assets
Fair value of plan assets at beginning of year	439,149	388,168
Actual (loss) return on plan assets	(29,016)	69,295
Employer contributions	45,219	20,829
Benefits paid	(36,222)	(39,143)
Fair value of plan assets at end of year	407,550	439,149	$ 388,168
Funded status at end of year	(147,245)	(174,962)
Amounts recognized in Consolidated Balance Sheets
Accrued benefit cost—current	(7,870)	(30,742)
Accrued benefit cost—noncurrent	$ (139,375)	$ (144,220)