Retirement Benefits Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Sep. 30, 2016	Dec. 31, 2016
Qualified Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 4,500
Components of net periodic benefit cost:
Service cost	5,425		$ 20,081
Interest cost	6,780		28,373
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer	600
Components of net periodic benefit cost:
Service cost	212		913
Interest cost	530		2,285
Other Benefit [Member]
Components of net periodic benefit cost:
Service cost	17		72
Interest cost	110		$ 500
Parent Company [Member] | Qualified Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	5,425	$ 5,018
Interest cost	6,780	7,099
Expected return on plan assets	(12,559)	(11,951)
Amortization of prior service cost	(495)	(495)
Amortization of net loss (gain)	462	0
Net periodic benefit cost	(387)	(329)
Parent Company [Member] | Supplemental Employee Retirement Plan [Member]
Components of net periodic benefit cost:
Service cost	212	228
Interest cost	530	571
Expected return on plan assets	0	0
Amortization of prior service cost	11	11
Amortization of net loss (gain)	394	269
Net periodic benefit cost	1,147	1,079
Parent Company [Member] | Other Benefit [Member]
Components of net periodic benefit cost:
Service cost	17	20
Interest cost	110	121
Expected return on plan assets	(117)	(115)
Amortization of prior service cost	0	0
Amortization of net loss (gain)	(86)	(113)
Net periodic benefit cost	(76)	(87)
Subsidiaries [Member] | Qualified Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	5,425	5,018
Interest cost	6,780	7,099
Expected return on plan assets	(12,569)	(11,970)
Amortization of prior service cost	(393)	(393)
Amortization of net loss (gain)	3,630	3,429
Net periodic benefit cost	2,873	3,183
Subsidiaries [Member] | Supplemental Employee Retirement Plan [Member]
Components of net periodic benefit cost:
Service cost	212	228
Interest cost	530	571
Expected return on plan assets	0	0
Amortization of prior service cost	11	11
Amortization of net loss (gain)	517	392
Net periodic benefit cost	1,270	1,202
Subsidiaries [Member] | Other Benefit [Member]
Components of net periodic benefit cost:
Service cost	17	20
Interest cost	110	121
Expected return on plan assets	(117)	(115)
Amortization of prior service cost	0	0
Amortization of net loss (gain)	(142)	(173)
Net periodic benefit cost	$ (132)	$ (147)