Schedule II - Financial Information of Registrant - Parent Company Balance Sheet (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Investments
Fixed-maturities available for sale—at fair value (amortized cost of $653,575 and $747,309)	$ 5,188,228	$ 5,017,711
Equity securities—at fair value (cost of $79,170 and $140,733)	89,057	148,965
Other invested assets—at fair value	8,625	5,511
Short-term investments—at fair value (includes $63,450 and $61,238 of reinvested cash collateral held under securities lending agreements)	660,566	402,090
Total investments	6,085,654	5,693,491
Cash	18,999	56,183
Investment in subsidiaries, at equity in net assets (Note C)	4,600,000
Other assets (Note D)	459,805	427,024
Total assets	7,593,933	7,063,729
Liabilities
Senior notes	1,417,781	1,413,504
Net deferred tax liability (Note A)	589,564	391,083
Other liabilities (Note D)	343,199	333,604
Total liabilities	3,196,128	3,144,402
Common stockholders’ equity
Common stock ($0.001 par value; 485,000 shares authorized; 2023: 173,247 and 153,179 shares issued and outstanding, respectively; 2022: 176,509 and 157,056 shares issued and outstanding, respectively)	173	176
Treasury stock, at cost (2023: 20,068 shares; 2022: 19,453 shares)	(945,870)	(930,643)
Additional paid-in capital	1,430,594	1,519,641
Retained earnings	4,243,759	3,786,952
Accumulated other comprehensive income (loss)	(330,851)	(456,799)
Total common stockholders’ equity	4,397,805	3,919,327	$ 4,258,796
Total liabilities and stockholders’ equity	7,593,933	7,063,729
Balance Sheet Parentheticals [Abstract]
Fixed-maturities available for sale, amortized cost	5,599,111	5,587,261
Trading securities, amortized cost	110,484	122,472
Equity securities, amortized cost	92,124	162,899
Reinvested cash collateral held under securities lending agreements	$ 149,364	$ 99,735
Common stock, par value (in usd per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	485,000	485,000
Common stock, shares, issued (in shares)	173,247	176,509
Common stock, shares, outstanding (in shares)	153,179	157,056	175,421	191,606
Treasury stock (in shares)	20,068	19,453
Parent Company
Investments
Fixed-maturities available for sale—at fair value (amortized cost of $653,575 and $747,309)	$ 623,957	$ 675,948
Equity securities—at fair value (cost of $79,170 and $140,733)	83,620	140,804
Other invested assets—at fair value	6,222	3,958
Short-term investments—at fair value (includes $63,450 and $61,238 of reinvested cash collateral held under securities lending agreements)	316,991	108,232
Total investments	1,030,790	928,942
Cash	2,200	1,872
Investment in subsidiaries, at equity in net assets (Note C)	4,990,822	4,576,644
Other assets (Note D)	165,612	158,027
Total assets	6,189,424	5,665,485
Liabilities
Senior notes	1,417,781	1,413,504
Net deferred tax liability (Note A)	226,141	196,829
Other liabilities (Note D)	147,697	135,825
Total liabilities	1,791,619	1,746,158
Common stockholders’ equity
Common stock ($0.001 par value; 485,000 shares authorized; 2023: 173,247 and 153,179 shares issued and outstanding, respectively; 2022: 176,509 and 157,056 shares issued and outstanding, respectively)	173	176
Treasury stock, at cost (2023: 20,068 shares; 2022: 19,453 shares)	(945,870)	(930,643)
Additional paid-in capital	1,430,594	1,519,641
Retained earnings	4,243,759	3,786,952
Accumulated other comprehensive income (loss)	(330,851)	(456,799)
Total common stockholders’ equity	4,397,805	3,919,327
Total liabilities and stockholders’ equity	6,189,424	5,665,485
Balance Sheet Parentheticals [Abstract]
Fixed-maturities available for sale, amortized cost	653,575	747,309
Equity securities, amortized cost	79,170	140,733
Reinvested cash collateral held under securities lending agreements	$ 63,450	$ 61,238