Defined Benefit Plan (Details) - USD ($)	3 Months Ended			6 Months Ended
	Jun. 30, 2020		Jun. 30, 2019	Jun. 30, 2020		Jun. 30, 2019	Dec. 31, 2019
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 2,337,000		$ 1,479,000	$ 4,780,000		$ 2,958,000
Interest cost	2,816,000		2,113,000	5,720,000		4,225,000
Other cost	(4,183,000)		1,117,000	(8,303,000)		2,235,000
Expected return on assets	1,169,000		(2,310,000)	2,668,000		(4,619,000)
Net periodic benefit cost	2,139,000		$ 2,399,000	4,865,000		$ 4,799,000
Plan Assets [Abstract]
Fair value of plan assets	238,585,000			238,585,000			$ 260,747,000
Employer Contributions [Abstract]
Estimated employer contributions for the current fiscal year	8,594,000			8,594,000
Defined Contribution Plan, Employer Discretionary Contribution Amount				0
Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	180,241,000			180,241,000			192,752,000
Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	58,344,000			58,344,000			67,995,000
Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0			0			0
Cash and cash equivalents
Plan Assets [Abstract]
Fair value of plan assets	6,237,000			6,237,000			11,801,000
Cash and cash equivalents | Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	6,237,000			6,237,000			11,801,000
Cash and cash equivalents | Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0			0			0
Cash and cash equivalents | Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0			0			0
Actively Managed | Equity securities (a)
Plan Assets [Abstract]
Fair value of plan assets	146,407,000	[1]		146,407,000	[1]		157,050,000	[2]
Actively Managed | Equity securities (a) | Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	142,839,000	[1]		142,839,000	[1]		149,265,000	[2]
Actively Managed | Equity securities (a) | Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	3,568,000	[1]		3,568,000	[1]		7,785,000	[2]
Actively Managed | Equity securities (a) | Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[1]		0	[2]
Fixed Income | Fixed income (b)
Plan Assets [Abstract]
Fair value of plan assets	85,941,000	[3]		85,941,000	[3]		91,896,000	[4]
Fixed Income | Fixed income (b) | Quoted Prices in Active Markets for Identical Assets
Plan Assets [Abstract]
Fair value of plan assets	31,165,000	[3]		31,165,000	[3]		31,686,000	[4]
Fixed Income | Fixed income (b) | Significant Observable Inputs
Plan Assets [Abstract]
Fair value of plan assets	54,776,000	[3]		54,776,000	[3]		60,210,000	[4]
Fixed Income | Fixed income (b) | Significant Unobservable Inputs
Plan Assets [Abstract]
Fair value of plan assets	$ 0	[3]		$ 0	[3]		$ 0	[4]

[1] Actively managed portfolio of equity securities with the goal to exceed the benchmark performance

[2]

[3] Actively managed portfolio of fixed income securities with the goal to exceed the benchmark performance

[4]