Retirement and Other Employee Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension Benefits
Net Periodic Pension Benefit Cost:
Service cost	$ 1,816	$ 1,596	$ 5,581	$ 4,850	$ 7,198	$ 6,713
Interest cost	2,702	2,648	7,708	8,114	10,413	10,631
Expected return on plan assets	(3,396)	(3,185)	(10,175)	(9,829)	(13,452)	(12,936)
Amortization of prior service cost	(157)	105	(472)	314	(368)	447
Amortization of net actuarial loss	2,835	1,660	7,525	4,958	9,178	7,569
Net periodic benefit cost	3,800	2,824	10,167	8,407	12,969	12,424
Pension contributions			21,400
SERP
Net Periodic Pension Benefit Cost:
Service cost	33	57	119	115	157	148
Interest cost	102	117	286	290	383	370
Amortization of prior service cost	(10)	(2)	(32)	(6)	(34)	(8)
Amortization of net actuarial loss	168	168	448	378	574	520
Net periodic benefit cost	293	340	821	777	1,080	1,030
OPEB
Net Periodic Pension Benefit Cost:
Service cost	945	737	2,785	1,950	3,435	2,686
Interest cost	1,175	1,109	3,502	3,270	4,592	4,226
Expected return on plan assets	(1,272)	(1,207)	(3,897)	(3,600)	(5,097)	(4,689)
Amortization of prior service cost	(253)	(253)	(758)	(758)	(1,011)	(1,011)
Amortization of net actuarial loss	699	268	2,061	725	2,303	1,291
Net periodic benefit cost	$ 1,294	$ 654	3,693	$ 1,587	$ 4,222	$ 2,503
Pension contributions			5,000
Employer contribution			$ 3,700
Healthcare benefits
Net Periodic Pension Benefit Cost:
Eligible age			55 years
Minimum service period for eligibility			5 years