Accounting Policies - Narrative (Details) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) company $ / shares	Jun. 30, 2017 USD ($) $ / shares	Jun. 30, 2018 USD ($) company $ / shares	Jun. 30, 2017 USD ($) $ / shares	Dec. 31, 2017 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of insurance companies based in the United States | company	6		6
Investment in variable interest entities	$ 32,200		$ 32,200		$ 32,100
Effective U.S. federal income tax expense rate	8.20%	6.50%	8.40%	7.20%
Percentage of available for sale securities	12.50%	12.30%	12.50%	12.30%
Net income	$ 16,984	$ 14,541	$ 32,617	$ 32,991
Basic earnings per share (in dollars per share) | $ / shares	$ 0.57	$ 0.49	$ 1.09	$ 1.12
Diluted earnings per share (in dollars per share) | $ / shares	$ 0.56	$ 0.48	$ 1.08	$ 1.09
ASU 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption of ASU	$ 0		$ 0
ASU 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption of ASU	0		0
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income			32,617	$ 32,991
Retained Earnings | ASU 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption of ASU	4,682		4,682
Net income	$ 451		$ (922)
Basic earnings per share (in dollars per share) | $ / shares	$ 0.02
Diluted earnings per share (in dollars per share) | $ / shares	$ 0.01
Reduction in basic and diluted earnings per share | $ / shares			$ 0.03
Retained Earnings | ASU 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption of ASU	$ (711)		$ (711)