Schedule II Condensed Financial Information of Registrant - Condensed Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
ASSETS
Investment in subsidiaries	$ 383,717	$ 328,516
Other assets	474,364	422,047
Total assets	12,478,704	11,031,896	$ 11,198,340
LIABILITIES AND SHAREHOLDERS' EQUITY
Short-term debt	135,000	0
Long-term debt	298,025	297,740
Other liabilities	384,173	290,358
Total liabilities	10,911,419	9,741,346
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, 2019, 66,088,808; 2018, 65,820,369	66	66
Additional paid-in capital	480,962	475,109
Retained earnings	1,352,539	1,216,582
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized investment gains on securities	230,448	96,941
Net funded status of pension benefit obligations	(10,767)	(12,185)
Treasury stock, at cost, 2019, 24,850,484 shares; 2018, 24,850,484 shares	(485,963)	(485,963)
Total shareholders' equity	1,567,285	1,290,550	$ 1,501,573
Total liabilities and shareholders' equity	12,478,704	11,031,896
Parent company
ASSETS
Investments and cash	1,453	5,255
Investment in subsidiaries	1,901,725	1,473,538
Other assets	62,442	66,138
Total assets	1,965,620	1,544,931
LIABILITIES AND SHAREHOLDERS' EQUITY
Short-term debt	135,000	0
Long-term debt	248,025	247,740
Other liabilities	15,310	6,641
Total liabilities	398,335	254,381
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, 2019, 66,088,808; 2018, 65,820,369	66	66
Additional paid-in capital	480,962	475,109
Retained earnings	1,352,539	1,216,582
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized investment gains on securities	230,448	96,941
Net funded status of pension benefit obligations	(10,767)	(12,185)
Treasury stock, at cost, 2019, 24,850,484 shares; 2018, 24,850,484 shares	(485,963)	(485,963)
Total shareholders' equity	1,567,285	1,290,550
Total liabilities and shareholders' equity	$ 1,965,620	$ 1,544,931