Investments (Fixed Maturities Securities Excluding Investments Classified as Trading) (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 13,620,095	$ 12,737,749
Allowance for Credit Losses	20,725	4,149
Fair Value	13,024,865	13,278,166
Fixed maturities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	13,620,095	12,737,749
Gross Unrealized Gain	185,680	658,544
Gross Unrealized Losses	760,185	113,978
Allowance for Credit Losses	20,725	4,149
Fair Value	13,024,865	13,278,166
Fixed maturities \| Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	50,525	18,684
Gross Unrealized Gain	727	1,501
Gross Unrealized Losses	1,402	5
Allowance for Credit Losses	0	0
Fair Value	49,850	20,180
Fixed maturities \| US Treasury securities and obligations of U.S. government authorities and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	302,835	303,040
Gross Unrealized Gain	1,239	31,011
Gross Unrealized Losses	7,071	111
Allowance for Credit Losses	0	0
Fair Value	297,003	333,940
Fixed maturities \| Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	604,286	584,244
Gross Unrealized Gain	17,194	46,978
Gross Unrealized Losses	12,308	701
Allowance for Credit Losses	0	0
Fair Value	609,172	630,521
Fixed maturities \| Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	333,824	324,454
Gross Unrealized Gain	10,697	29,299
Gross Unrealized Losses	17,993	3,271
Allowance for Credit Losses	399	11
Fair Value	326,129	350,471
Fixed maturities \| U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,973,499	4,794,878
Gross Unrealized Gain	115,899	366,764
Gross Unrealized Losses	316,620	29,770
Allowance for Credit Losses	0	0
Fair Value	4,772,778	5,131,872
Fixed maturities \| U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,136,230	1,964,767
Gross Unrealized Gain	15,758	59,037
Gross Unrealized Losses	100,031	16,880
Allowance for Credit Losses	2,482	2,049
Fair Value	2,049,475	2,004,875
Fixed maturities \| Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	994,201	906,031
Gross Unrealized Gain	8,921	34,234
Gross Unrealized Losses	68,260	10,363
Allowance for Credit Losses	1,177	0
Fair Value	933,685	929,902
Fixed maturities \| Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,001,495	2,741,449
Gross Unrealized Gain	8,711	62,932
Gross Unrealized Losses	204,953	48,381
Allowance for Credit Losses	16,667	2,089
Fair Value	2,788,586	2,753,911
Fixed maturities \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	624,560	547,549
Gross Unrealized Gain	486	860
Gross Unrealized Losses	7,251	1,099
Allowance for Credit Losses	0	0
Fair Value	617,795	547,310
Fixed maturities \| Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	598,640	552,653
Gross Unrealized Gain	6,048	25,928
Gross Unrealized Losses	24,296	3,397
Allowance for Credit Losses	0	0
Fair Value	$ 580,392	$ 575,184

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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