Significant Accounting Policies and Pronouncements (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019		Jan. 01, 2020	Dec. 31, 2018		Jan. 01, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Commercial mortgage and other loans, Loan-to- value ratios (greater than)	100.00%
Commercial mortgage and other loans, Loan-to-value ratios (less than)	100.00%
Commercial mortgage and other loans, Debt service coverage ratios (less than)	1.0
Commercial mortgage and other loans, Debt service coverage ratios (greater than)	1.0
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings / (accumulated deficit)	$ (3,217,350)	$ (46,693)
Accumulated other comprehensive income (loss)	1,533,059	428,612
Deposit assets related to a reinsurance agreement	$ 425,900	67,300
Uncertain tax positions measurement percentage (greater than)	50.00%
Securities Loaned Transactions Collateral Fair Value of Domestic Securities	102.00%
Securities Loaned Transactions Collateral Fair Value of Foreign Securities	105.00%
Funds withheld liability for assets retained under a reinsurance agreement	$ 386,000	$ 60,400
Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Repurchase and Resale Agreements, Collateral, Percentage	95.00%
Insurance revenue and expense recognition, Assumed interest rate	0.00%
Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Insurance revenue and expense recognition, Assumed interest rate		8.30%
ASU 2016-13 | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings / (accumulated deficit)				$ 1,400
Accumulated Other Comprehensive   Income (loss)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated other comprehensive income (loss)	$ 1,533,059	$ 428,612			$ (324,373)			$ (90,124)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	1,533,059	428,612			(324,373)			(90,124)
Accumulated Other Comprehensive   Income (loss) | ASU 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest							$ 36,700	(36,714)
Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ (3,217,350)	(46,693)			943,005			(776,762)
Retained earnings | ASU 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest							(36,700)	$ 36,714
Retained earnings | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		$ (1,392)	[1]		$ (371)	[2]
Restatement Adjustment | ASU 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings / (accumulated deficit)							337
Accumulated other comprehensive income (loss)							(3)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest							$ 334

[1] Includes the impact from the adoption of ASU 2016-13. See Note 2.

[2] Includes the impact from the adoption of ASU 2017-08 and 2017-12.