Schedule II Financial Information of Registrant - Parent Company Balance Sheet (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets
Fixed-maturities available for sale—at fair value (amortized cost of $836,191 and $429,999)	$ 5,723,340	$ 4,688,911
Trading securities—at fair value (amortized cost of $— and $—)	290,885	317,150
Equity securities—at fair value (cost of $— and $13,280)	151,240	130,221
Short-term investments—at fair value	618,004	518,393
Other invested assets—at fair value	4,973	4,072
Total investments	6,788,442	5,658,747
Cash	87,915	92,729
Investment in subsidiaries, at equity in net assets (Note C)	4,100,000
Accounts and notes receivable	121,294	93,630
Other assets (Note C)	447,447	409,672
Total assets	7,948,021	6,808,313
Liabilities and Stockholders’ Equity
Senior notes	1,405,674	887,110
Net deferred tax liability (Note A)	213,897	71,084
Other liabilities	291,855	343,105
Total liabilities	3,663,668	2,759,590
Common stock: par value $0.001 per share; 485,000 shares authorized at December 31, 2020 and 2019; 210,130 and 219,123 shares issued at December 31, 2020 and 2019, respectively; 191,606 and 201,164 shares outstanding at December 31, 2020 and 2019, respectively	210	219
Treasury stock, at cost: 18,524 and 17,959 shares at December 31, 2020 and 2019, respectively	(910,115)	(901,657)
Additional paid-in capital	2,245,897	2,449,884
Retained earnings	2,684,636	2,389,789
Accumulated other comprehensive income (loss)	263,725	110,488
Total common stockholders’ equity	4,284,353	4,048,723
Total liabilities and stockholders’ equity	7,948,021	6,808,313
Balance Sheet Parentheticals [Abstract]
Fixed-maturities available for sale—amortized cost	5,393,623	4,549,534
Trading Securities, Amortized Cost	260,773	297,505
Equity Securities, Amortized Cost	145,501	125,311
Parent Company
Assets
Fixed-maturities available for sale—at fair value (amortized cost of $836,191 and $429,999)	844,393	430,442
Equity securities—at fair value (cost of $— and $13,280)	0	13,381
Short-term investments—at fair value	233,569	162,363
Other invested assets—at fair value	3,000	1,500
Total investments	1,080,962	607,686
Cash	20,141	23,534
Investment in subsidiaries, at equity in net assets (Note C)	4,545,508	4,413,065
Accounts and notes receivable	300,656	100,775
Other assets (Note C)	75,305	113,917
Total assets	6,022,572	5,258,977
Liabilities and Stockholders’ Equity
Senior notes	1,405,674	887,110
Net deferred tax liability (Note A)	272,868	253,739
Other liabilities	59,677	69,405
Total liabilities	1,738,219	1,210,254
Common stock: par value $0.001 per share; 485,000 shares authorized at December 31, 2020 and 2019; 210,130 and 219,123 shares issued at December 31, 2020 and 2019, respectively; 191,606 and 201,164 shares outstanding at December 31, 2020 and 2019, respectively	210	219
Treasury stock, at cost: 18,524 and 17,959 shares at December 31, 2020 and 2019, respectively	(910,115)	(901,657)
Additional paid-in capital	2,245,897	2,449,884
Retained earnings	2,684,636	2,389,789
Accumulated other comprehensive income (loss)	263,725	110,488
Total common stockholders’ equity	4,284,353	4,048,723
Total liabilities and stockholders’ equity	$ 6,022,572	$ 5,258,977
Balance Sheet Parentheticals [Abstract]
Common Stock, Par or Stated Value Per Share	$ 0.001	$ 0.001
Common Stock, Shares Authorized	485,000,000	485,000,000
Common Stock, Shares, Issued	210,130,000	219,123,000
Common Stock, Shares, Outstanding	191,606,000	201,164,000
Treasury Stock, Shares	18,524,000	17,959,000
Fixed-maturities available for sale—amortized cost	$ 836,191	$ 429,999
Equity Securities, Amortized Cost	$ 0	$ 13,280