Pension and Other Retirement Plans - Defined Benefit Pension Plans and Other Post Retirement Plans Summarized Info (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligations, beginning of year	$ 1,990,263	$ 1,986,971
Service cost	768	692	$ 709
Interest cost	78,195	82,724	81,815
Plan amendments	601	1,025
Impact of Medicare Reform Act	0	0
Actuarial loss (gain)	94,092	22,615
Benefits paid	(107,043)	(103,764)
Projected benefit obligations, end of year	2,056,876	1,990,263	1,986,971
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plans’ assets, beginning of year	1,545,862	1,530,898
Actual return on plans’ assets	221,182	118,728
Employer contributions	0	0
Benefits paid	(107,043)	(103,764)
Fair value of plans’ assets, end of year	1,660,001	1,545,862	1,530,898
Under funded status of the plans	(396,875)	(444,401)
Other Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligations, beginning of year	8,875	10,055
Service cost	0	0	81
Interest cost	264	320	451
Plan amendments	0	0	(4,000)
Impact of Medicare Reform Act	42	60
Actuarial loss (gain)	(593)	(496)
Benefits paid	(914)	(1,064)
Projected benefit obligations, end of year	7,674	8,875	10,055
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plans’ assets, beginning of year	0	0
Actual return on plans’ assets	0	0
Employer contributions	914	1,064
Benefits paid	(914)	(1,064)
Fair value of plans’ assets, end of year	0	0	$ 0
Under funded status of the plans	$ (7,674)	$ (8,875)