Schedule II Condensed Financial Information of Registrant (Details) - USD ($) $ in Thousands		Dec. 31, 2016		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
ASSETS
Other assets		$ 321,874		$ 292,139
Total assets		10,576,824		10,056,972	$ 9,768,346
LIABILITIES AND SHAREHOLDERS' EQUITY
Long-term debt		247,209		246,975
Other liabilities		378,620		368,559
Total liabilities		9,282,842		8,792,311
Preferred stock, $0.001 par value, authorized 1,000,000 shares; none issued		0		0
Common stock, $0.001 par value, authorized 75,000,000 shares;issued, 2016, 64,917,683; 2015, 64,537,554		65		65
Additional paid-in capital		453,479		442,648
Retained earnings		1,155,732		1,116,277
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized investment gains on fixed maturities and equity securities	[1],[2]	175,738		175,167	297,554	$ 133,990
Net funded status of pension benefit obligations	[1]	11,817		11,794	12,953	$ 11,776
Treasury stock, at cost, 2016, 24,672,932 shares; 2015, 23,971,522 shares		(479,215)		(457,702)
Total shareholders' equity		1,293,982		1,264,661	$ 1,336,463
Total liabilities and shareholders' equity		10,576,824		10,056,972
Parent Company [Member]
ASSETS
Investments and cash		4,069	[1]	13,237
Investment in subsidiaries		1,487,457	[1]	1,451,290
Other assets		60,057	[1]	57,743
Total assets		1,551,583	[1]	1,522,270
LIABILITIES AND SHAREHOLDERS' EQUITY
Long-term debt		247,209	[1]	246,975
Other liabilities		10,392	[1]	10,634
Total liabilities		257,601	[1]	257,609
Preferred stock, $0.001 par value, authorized 1,000,000 shares; none issued		0	[1]	0
Common stock, $0.001 par value, authorized 75,000,000 shares;issued, 2016, 64,917,683; 2015, 64,537,554		65	[1]	65
Additional paid-in capital		453,479	[1]	442,648
Retained earnings		1,155,732	[1]	1,116,277
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized investment gains on fixed maturities and equity securities		175,738	[1]	175,167
Net funded status of pension benefit obligations		(11,817)	[1]	(11,794)
Treasury stock, at cost, 2016, 24,672,932 shares; 2015, 23,971,522 shares		(479,215)	[1]	(457,702)
Total shareholders' equity		1,293,982	[1]	1,264,661
Total liabilities and shareholders' equity		$ 1,551,583	[1]	$ 1,522,270

[1] All amounts are net of tax.

[2] The pretax amounts reclassified from accumulated other comprehensive income, $5,176, $11,667 and $10,943, are included in net realized investment gains and losses and the related tax expenses, $1,812, $4,083 and $3,830, are included in income tax expense in the Consolidated Statements of Operations for the years ended December 31, 2016, 2015 and 2014, respectively.