Derivative Instruments Derivative Instruments (Balance Sheet Offsetting) (Details) - USD ($) $ in Thousands		Mar. 31, 2020	Dec. 31, 2019
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement		$ 250,450	$ 105,674
Derivative Asset, Amount Offset by Liabiilty		0	0
Derivative Assets	[1]	250,450	105,674
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement		20,443	10,219
Derivative Liability, Amount Offset by Asset		0	0
Derivative Liabilities	[1]	20,443	10,219
Derivative Subject to Master Netting Agreement [Member]
Derivative Asset, Fair Value, Gross Asset		250,215	105,147
Derivative Asset, Amount Offset by Liabiilty		0	0
Derivative Asset, Net Amount Presented in the Balance Sheet		250,215	105,147
Derivative Asset, Not Offset, Policy Election Deduction		(17,394)	(8,104)
derivatve asset, fair value of collateral		(130,611)	(59,525)
Derivative Asset, Fair Value, Amount Offset Against Collateral		102,210	37,518
Derivative Liability, Fair Value, Gross Liability		20,148	10,083
Derivative Liability, Amount Offset by Asset		0	0
Derivative Liability, Net Amount Presented in the Balance Sheet		20,148	10,083
Derivative Liability, Not Offset, Policy Election Deduction		(17,394)	(8,104)
Derivative Liability, Fair Value of Collateral		(1,443)	(437)
Derivative Liability, Fair Value, Amount Offset Against Collateral		1,311	1,542
Derivative Not Subject to Master Netting Agreement [Member]
Derivative Asset, Fair Value, Gross Asset		235	527
Derivative Asset, Amount Offset by Liabiilty		0	0
Derivative Asset, Not Subject to Master Netting Arrangement		235	527
Derivative Liability, Fair Value, Gross Liability		295	136
Derivative Liability, Amount Offset by Asset		0	0
Derivative Liability, Not Subject to Master Netting Arrangement		$ 295	$ 136

[1] * The fair value of each class of derivative is shown in Note 11.