Schedule II Condensed Financial Information of Registrant - Condensed Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
ASSETS
Investment in subsidiaries	$ 328,516	$ 247,266
Other assets	422,047	381,182
Total assets	11,031,896	11,198,340	$ 10,576,824
LIABILITIES AND SHAREHOLDERS' EQUITY
Long-term debt	297,740	297,469
Other liabilities	290,358	341,053
Total liabilities	9,741,346	9,696,767
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, 2018, 65,820,369; 2017, 65,439,245	66	65
Additional paid-in capital	475,109	464,246
Retained earnings	1,216,582	1,231,177
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized investment gains on securities	96,941	300,177
Net funded status of pension benefit obligations	(12,185)	(13,217)
Treasury stock, at cost, 2018, 24,850,484 shares; 2017, 24,721,372 shares	(485,963)	(480,875)
Total shareholders' equity	1,290,550	1,501,573	$ 1,293,982
Total liabilities and shareholders' equity	11,031,896	11,198,340
Parent Company [Member]
ASSETS
Investments and cash	5,255	6,464
Investment in subsidiaries	1,473,538	1,685,390
Other assets	66,138	66,445
Total assets	1,544,931	1,758,299
LIABILITIES AND SHAREHOLDERS' EQUITY
Long-term debt	247,740	247,469
Other liabilities	6,641	9,257
Total liabilities	254,381	256,726
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, 2018, 65,820,369; 2017, 65,439,245	66	65
Additional paid-in capital	475,109	464,246
Retained earnings	1,216,582	1,231,177
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized investment gains on securities	96,941	300,177
Net funded status of pension benefit obligations	(12,185)	(13,217)
Treasury stock, at cost, 2018, 24,850,484 shares; 2017, 24,721,372 shares	(485,963)	(480,875)
Total shareholders' equity	1,290,550	1,501,573
Total liabilities and shareholders' equity	$ 1,544,931	$ 1,758,299