Benefit Plans and Other Postretirement Benefits, Changes in Level 3 Postretirement Benefit Plan Assets (Details) (Postretirement Benefit Plan, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2014
Changes in Level 3 Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1				$ 425,282
Fair value of plan assets at Dec. 31	438,193		425,917	425,282
Level 3
Changes in Level 3 Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1	36,064		31,602	0
Net realized gains (losses)	0		(934)
Net unrealized gains (losses)	0		5,068
Purchases, issuances and settlements, net	0		328
Transfers in (out) of Level 3	(36,064)	[1]	0
Fair value of plan assets at Dec. 31	0		36,064	0
Asset-backed securities
Changes in Level 3 Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1				3,240
Fair value of plan assets at Dec. 31	2,991		670	3,240
Asset-backed securities | Level 3
Changes in Level 3 Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1	670		6,941	0
Net realized gains (losses)	0		(293)
Net unrealized gains (losses)	0		1,669
Purchases, issuances and settlements, net	0		(7,647)
Transfers in (out) of Level 3	(670)	[1]	0
Fair value of plan assets at Dec. 31	0		670	0
Mortgage-backed securities
Changes in Level 3 Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1				10,071
Fair value of plan assets at Dec. 31	21,593			10,071
Mortgage-backed securities | Level 3
Changes in Level 3 Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1	35,394		24,038	0
Net realized gains (losses)	0		(641)
Net unrealized gains (losses)	0		3,429
Purchases, issuances and settlements, net	0		8,568
Transfers in (out) of Level 3	(35,394)	[1]	0
Fair value of plan assets at Dec. 31	0		35,394	0
Private equity investments | Level 3
Changes in Level 3 Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1			479
Net realized gains (losses)			0
Net unrealized gains (losses)			(65)
Purchases, issuances and settlements, net			(414)
Transfers in (out) of Level 3			0
Fair value of plan assets at Dec. 31			0
Real estate | Level 3
Changes in Level 3 Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1			144
Net realized gains (losses)			0
Net unrealized gains (losses)			35
Purchases, issuances and settlements, net			(179)
Transfers in (out) of Level 3			0
Fair value of plan assets at Dec. 31			$ 0

[1] Transfers out of Level 3 into Level 2 were principally due to diminished use of unobservable inputs that were previously significant to these fair value measurements and were subsequently sold during 2013.