Retirement and Other Employee Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2016	Jun. 30, 2016	Jun. 30, 2015
Pension Benefits
Net Periodic Pension Benefit Cost:
Service cost	$ 1,949	$ 1,883	$ 3,899	$ 3,765		$ 7,575	$ 6,978
Interest cost	2,633	2,503	5,266	5,006		10,538	10,359
Expected return on plan assets	(3,465)	(3,390)	(6,931)	(6,779)		(13,719)	(13,241)
Amortization of prior service cost	(157)	(157)	(315)	(315)		(630)	(106)
Amortization of net actuarial loss	2,106	2,345	4,213	4,690		9,556	8,003
Net periodic benefit cost	3,066	3,184	6,132	6,367		13,320	11,993
Pension contributions					$ 12,400
Employer contribution			6,100
SERP
Net Periodic Pension Benefit Cost:
Service cost	45	43	91	85		164	180
Interest cost	108	92	215	184		413	399
Amortization of prior service cost	(3)	(11)	(7)	(21)		(28)	(26)
Amortization of net actuarial loss	139	140	278	280		595	574
Net periodic benefit cost	289	264	577	528		1,144	1,127
OPEB
Net Periodic Pension Benefit Cost:
Service cost	814	920	1,627	1,840		3,500	3,227
Interest cost	1,157	1,163	2,313	2,327		4,656	4,525
Expected return on plan assets	(1,374)	(1,312)	(2,748)	(2,625)		(5,320)	(5,032)
Amortization of prior service cost	(253)	(253)	(506)	(506)		(1,011)	(1,011)
Amortization of net actuarial loss	257	681	515	1,363		1,901	1,873
Net periodic benefit cost	$ 601	$ 1,199	1,201	$ 2,399		$ 3,726	$ 3,582
Pension contributions					$ 2,600
Employer contribution			$ 1,200
Healthcare benefits
Net Periodic Pension Benefit Cost:
Eligible age			55 years
Minimum service period for eligibility			5 years