Benefit Plans and Other Postretirement Benefits, Changes in Level 3 Postretirement Benefit Plan Assets (Details) - Postretirement Benefit Plan - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Changes in Level 3 Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1		$ 425,282	$ 438,193
Fair value of plan assets at Dec. 31		399,442	425,282	$ 438,193
Level 3
Changes in Level 3 Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1		0	0	36,064
Net realized gains (losses)				0
Net unrealized gains (losses)				0
Purchases, issuances and settlements, net				0
Transfers in (out) of Level 3	[1]			(36,064)
Fair value of plan assets at Dec. 31		0	0	0
Asset-backed securities
Changes in Level 3 Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1		3,240	0
Fair value of plan assets at Dec. 31		25,602	3,240	0
Asset-backed securities | Level 3
Changes in Level 3 Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1		0		670
Net realized gains (losses)				0
Net unrealized gains (losses)				0
Purchases, issuances and settlements, net				0
Transfers in (out) of Level 3	[1]			(670)
Fair value of plan assets at Dec. 31		0	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		31,778	10,071
Mortgage-backed securities | Level 3
Changes in Level 3 Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1		0	0	35,394
Net realized gains (losses)				0
Net unrealized gains (losses)				0
Purchases, issuances and settlements, net				0
Transfers in (out) of Level 3	[1]			(35,394)
Fair value of plan assets at Dec. 31		$ 0	$ 0	$ 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.