Retirement Plans (Tables)

6 Months Ended

Jun. 30, 2017

Retirement Plans [Abstract]

Components of Net Periodic Benefits Cost

					Other Post-
(in thousands)	Pension Benefits				retirement Benefits
Three months ended June 30,	2017		2016		2017		2016
Service cost	$	4,093	$	3,611	$	47	$	45
Interest cost		4,291		4,023		179		203
Expected return on plan assets		(9,576)		(8,554)		—		—
Amortization of net loss		1,766		1,426		(48)		53
Net periodic benefit cost	$	574	$	506	$	178	$	301
Six months ended June 30, 2017
Service cost	$	7,843	$	6,876	$	95	$	74
Interest cost		8,414		9,011		359		409
Expected return on plan assets		(18,326)		(17,445)		—		—
Amortization of net loss		3,201		2,894		(96)		22
Net periodic benefit cost	$	1,132	$	1,336	$	358	$	505