Schedule II Condensed Financial Information of Registrant (Details) - USD ($) $ in Thousands		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
ASSETS
Other assets		$ 294,510	$ 277,350
Total assets		10,059,343	9,768,527	$ 8,826,672
LIABILITIES AND SHAREHOLDERS’ EQUITY
Long-term debt, current and noncurrent		249,346	199,939
Other liabilities		368,559	422,362
Total liabilities		8,794,682	8,432,064
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, 2015, 64,537,554; 2014, 64,245,048		65	64
Additional paid-in capital		442,648	422,232
Retained earnings		1,116,277	1,065,318
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized gains on fixed maturities and equity securities	[1],[2]	175,167	297,554	133,990	$ 382,400
Net funded status of pension and other postretirement benefit obligations	[1]	(11,794)	(12,953)	(11,776)	$ (15,311)
Treasury stock, at cost, 2015, 23,971,522 shares; 2014, 23,308,430 shares		(457,702)	(435,752)
Total shareholders' equity		1,264,661	1,336,463	$ 1,099,305
Total liabilities and shareholders' equity		10,059,343	9,768,527
Parent Company [Member]
ASSETS
Investments and cash		13,237	28,640
Investment in subsidiaries		1,451,290	1,515,769
Other assets		60,114	58,365
Total assets		1,524,641	1,602,774
LIABILITIES AND SHAREHOLDERS’ EQUITY
Short-term debt		0	38,000
Long-term debt, current and noncurrent		249,346	199,939
Other liabilities		10,634	28,372
Total liabilities		259,980	266,311
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, 2015, 64,537,554; 2014, 64,245,048		65	64
Additional paid-in capital		442,648	422,232
Retained earnings		1,116,277	1,065,318
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized gains on fixed maturities and equity securities		175,167	297,554
Net funded status of pension and other postretirement benefit obligations		(11,794)	(12,953)
Treasury stock, at cost, 2015, 23,971,522 shares; 2014, 23,308,430 shares		(457,702)	(435,752)
Total shareholders' equity		1,264,661	1,336,463
Total liabilities and shareholders' equity		$ 1,524,641	$ 1,602,774

[1] All amounts are net of tax.

[2] The pretax amounts reclassified from accumulated other comprehensive income, $11,667, $10,943 and $22,245, are included in net realized investment gains and losses and the related tax expenses, $4,083, $3,830 and $7,786, are included in income tax expense in the Consolidated Statements of Operations for the years ended December 31, 2015, 2014 and 2013, respectively.