Retirement and Other Employee Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Pension Benefits
Net Periodic Pension Benefit Cost:
Service cost	$ 1,949	$ 1,883	$ 7,508	$ 6,722
Interest cost	2,633	2,503	10,408	10,589
Expected return on plan assets	(3,465)	(3,390)	(13,643)	(13,173)
Amortization of prior service cost	(157)	(157)	(630)	156
Amortization of net actuarial loss	2,106	2,345	9,795	7,308
Net periodic benefit cost	3,066	3,184	13,438	11,602
Pension contributions	12,400
SERP
Net Periodic Pension Benefit Cost:
Service cost	45	43	161	167
Interest cost	108	92	398	393
Amortization of prior service cost	(3)	(11)	(35)	(17)
Amortization of net actuarial loss	139	140	596	539
Net periodic benefit cost	289	264	1,120	1,082
OPEB
Net Periodic Pension Benefit Cost:
Service cost	813	920	3,606	2,914
Interest cost	1,156	1,164	4,663	4,442
Expected return on plan assets	(1,374)	(1,312)	(5,258)	(4,916)
Amortization of prior service cost	(253)	(253)	(1,011)	(1,011)
Amortization of net actuarial loss	258	681	2,324	1,420
Net periodic benefit cost	600	$ 1,200	$ 4,324	$ 2,849
Pension contributions	$ 2,600
Healthcare benefits
Net Periodic Pension Benefit Cost:
Eligible age	55 years
Minimum service period for eligibility	5 years