Derivative Instruments Derivative Instruments (Balance Sheet Offsetting) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Balance Sheet Offsetting [Line Items]
Derivative Assets	$ 41,210	$ 8,349
Derivative Asset, Amount Offset by Liabiilty	0	0
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	41,210	8,349
Derivative Liability	13,421	8,074
Derivative Liability, Amount Offset by Asset	0	0
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	13,421	8,074
Derivative Subject to Master Netting Agreement [Member]
Balance Sheet Offsetting [Line Items]
Derivative Asset, Fair Value, Gross Asset	40,613	7,726
Derivative Asset, Amount Offset by Liabiilty	0	0
Derivative Asset, Noncurrent	40,613	7,726
Derivative Asset, Not Offset, Policy Election Deduction	(2,992)	(233)
Derivative Asset, Fair Value of Collateral	(26,174)	(824)
Derivative Asset, Fair Value, Amount Offset Against Collateral	11,447	6,669
Derivative Liability, Fair Value, Gross Liability	13,333	7,935
Derivative Liability, Amount Offset by Asset	0	0
Derivative Liability, Noncurrent	13,333	7,935
Derivative Liability, Not Offset, Policy Election Deduction	(2,992)	(233)
Derivative Liability, Fair Value of Collateral	(261)	(1,570)
Derivative Liability, Fair Value, Amount Offset Against Collateral	10,080	6,132
Derivative Not Subject to Master Netting Agreement [Member]
Balance Sheet Offsetting [Line Items]
Derivative Asset, Fair Value, Gross Asset	597	623
Derivative Asset, Amount Offset by Liabiilty	0	0
Derivative Asset, Not Subject to Master Netting Arrangement	597	623
Derivative Liability, Fair Value, Gross Liability	88	139
Derivative Liability, Amount Offset by Asset	0	0
Derivative Liability, Not Subject to Master Netting Arrangement	$ 88	$ 139