Pension Benefit and Other Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Details) (USD $)	4 Months Ended	3 Months Ended		6 Months Ended		3 Months Ended			6 Months Ended
	Apr. 30, 2013	Jun. 30, 2013 Pension Benefits [Member]	Jun. 30, 2012 Pension Benefits [Member]	Jun. 30, 2013 Pension Benefits [Member]	Jun. 30, 2012 Pension Benefits [Member]	Jun. 30, 2013 Retirement Health and Life Insurance Benefits [Member]	Jun. 30, 2012 Retirement Health and Life Insurance Benefits [Member]	Mar. 31, 2012 Retirement Health and Life Insurance Benefits [Member]	Jun. 30, 2013 Retirement Health and Life Insurance Benefits [Member]	Jun. 30, 2012 Retirement Health and Life Insurance Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 956,000	$ 1,108,000	$ 2,210,000	$ 2,297,000	$ 170,000	$ 149,000		$ 341,000	$ 331,000
Interest cost		1,954,000	2,124,000	4,015,000	4,242,000	71,000	92,000		141,000	180,000
Expected return on plan assets	(0.086)	(2,731,000)	(2,495,000)	(5,447,000)	(4,903,000)
Amortization of prior service cost		31,000	116,000	124,000	232,000	(58,000)	(113,000)		(115,000)	(226,000)
Amortization of net loss		1,049,000	1,442,000	2,525,000	2,791,000	83,000	66,000		165,000	181,000
Special termination benefit							(707,000)	2,300,000		1,593,000
Curtailment charge		1,537,000		1,537,000
Net periodic benefit cost		$ 2,796,000	$ 2,295,000	$ 4,964,000	$ 4,659,000	$ 266,000	$ (513,000)		$ 532,000	$ 2,059,000