Schedule II Financial Information of Registrant Parent Company Balance Sheet (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets
Fixed-maturities available for sale—at fair value	$ 3,458,719	$ 2,838,512
Short-term investments—at fair value	415,658	741,531
Total investments	4,643,942	4,462,430
Cash	80,569	52,149
Restricted cash (Note B)	15,675	9,665
Investment in subsidiaries, at equity in net assets (Note C)	3,600,000
Federal income taxes recoverable, net—current	48,400
Other assets (Note E)	407,849	343,835
Total assets	5,900,881	5,863,174
Liabilities and Stockholders’ Equity
Long-term debt (Note F)	1,027,074	1,069,537
Other liabilities	353,845	321,859
Total liabilities	2,900,843	2,990,888
Common stock: par value $.001 per share; 485,000,000 shares authorized at December 31, 2017 and 2016; 233,416,989 and 232,091,921 shares issued at December 31, 2017 and 2016, respectively; 215,814,188 and 214,521,079 shares outstanding at December 31, 2017 and 2016, respectively	233	232
Treasury stock, at cost: 17,602,801 and 17,570,842 shares at December 31, 2017 and 2016, respectively	(893,888)	(893,332)
Additional paid-in capital	2,754,275	2,779,891
Retained earnings	1,116,333	997,890
Accumulated other comprehensive income (loss)	23,085	(12,395)
Total common stockholders’ equity	3,000,038	2,872,286
Liabilities and Equity	5,900,881	5,863,174
Balance Sheet Parentheticals [Abstract]
Property and Equipment, Owned, Accumulated Depreciation	106,000	118,500
Parent Company [Member]
Assets
Fixed-maturities available for sale—at fair value	10,785	79,336
Short-term investments—at fair value	83,356	311,418
Total investments	94,141	390,754
Cash	13,173	8,256
Restricted cash (Note B)	0	124
Investment in subsidiaries, at equity in net assets (Note C)	3,764,865	3,383,089
Accounts and notes receivable (Note D)	103,561	305,316
Federal income taxes recoverable, net—current	35,741	0
Other assets (Note E)	166,051	166,098
Total assets	4,177,532	4,253,637
Liabilities and Stockholders’ Equity
Long-term debt (Note F)	1,027,074	1,069,537
Federal income taxes—current	0	78,980
Federal income taxes—deferred	97,067	187,309
Other liabilities	53,353	45,525
Total liabilities	1,177,494	1,381,351
Common stock: par value $.001 per share; 485,000,000 shares authorized at December 31, 2017 and 2016; 233,416,989 and 232,091,921 shares issued at December 31, 2017 and 2016, respectively; 215,814,188 and 214,521,079 shares outstanding at December 31, 2017 and 2016, respectively	233	232
Treasury stock, at cost: 17,602,801 and 17,570,842 shares at December 31, 2017 and 2016, respectively	(893,888)	(893,332)
Additional paid-in capital	2,754,275	2,779,891
Retained earnings	1,116,333	997,890
Accumulated other comprehensive income (loss)	23,085	(12,395)
Total common stockholders’ equity	3,000,038	2,872,286
Liabilities and Equity	$ 4,177,532	$ 4,253,637
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	233,416,989	232,091,921
Common Stock, Shares, Outstanding	215,814,188	214,521,079
Treasury Stock, Shares	17,602,801	17,570,842