Schedule II Condensed Financial Information of Registrant - Condensed Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
ASSETS
Other assets	$ 381,182	$ 321,874
Total assets	11,198,340	10,576,824	$ 10,056,972
LIABILITIES AND SHAREHOLDERS' EQUITY
Long-term debt	297,469	247,209
Other liabilities	341,053	378,620
Total liabilities	9,696,767	9,282,842
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, 2017, 65,439,245; 2016, 64,917,683	65	65
Additional paid-in capital	464,246	453,479
Retained earnings	1,231,177	1,155,732
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized investment gains on fixed maturity and equity securities	300,177	175,738
Net funded status of pension benefit obligations	(13,217)	(11,817)
Treasury stock, at cost, 2017, 24,721,372 shares; 2016, 24,672,932 shares	(480,875)	(479,215)
Total shareholders' equity	1,501,573	1,293,982	$ 1,264,661
Total liabilities and shareholders' equity	11,198,340	10,576,824
Parent Company [Member]
ASSETS
Investments and cash	6,464	4,069
Investment in subsidiaries	1,685,390	1,487,457
Other assets	66,445	60,057
Total assets	1,758,299	1,551,583
LIABILITIES AND SHAREHOLDERS' EQUITY
Long-term debt	247,469	247,209
Other liabilities	9,257	10,392
Total liabilities	256,726	257,601
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, 2017, 65,439,245; 2016, 64,917,683	65	65
Additional paid-in capital	464,246	453,479
Retained earnings	1,231,177	1,155,732
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized investment gains on fixed maturity and equity securities	300,177	175,738
Net funded status of pension benefit obligations	(13,217)	(11,817)
Treasury stock, at cost, 2017, 24,721,372 shares; 2016, 24,672,932 shares	(480,875)	(479,215)
Total shareholders' equity	1,501,573	1,293,982
Total liabilities and shareholders' equity	$ 1,758,299	$ 1,551,583