Investments (Narrative) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) Security Business investment_interest Issuer	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) Security Issuer	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Number of investment affiliates exceeding shareholder's equity ten percent threshold limit | investment_interest	0
Threshold limit of investments based on shareholders' equity (percent)	10.00%
Securities on deposit with state insurance departments	$ 45,800
Business owned life insurance cost	33,000
Fair Value	$ 297,309		$ 230,889
Number of LPs / LLCs with investment ownership percent over 25% | Business	3
Investment in unconsolidated subsidiaries	$ 401,030		330,591
Equity in earnings, losses from affordable housing projects	(4,100)	$ (3,300)
Affordable housing tax credits	4,600	4,600
Losses from historic tax credit investments	(1,900)	(400)
Tax credits recognized related to historic tax credit investments	700	1,800
Net impairment losses recognized in earnings	0	171
Portion of noncredit OTTI losses recognized in OCI	0	248
Credit-related other-than-temporary impairment recognized in earnings	$ 1,313	$ 1,329	1,313	$ 1,158
Tax Credit Partnerships Almost 100% Ownership
Schedule of Available-for-sale Securities [Line Items]
Number of tax credit partnerships almost 100% ownership percentage | investment_interest	2
Investment in unconsolidated subsidiaries	$ 30,700		32,500
Tax Credit Partnerships Less Than 20% Ownership
Schedule of Available-for-sale Securities [Line Items]
Investment in unconsolidated subsidiaries	49,800		52,100
Other Limited Partnerships and Limited Liability Company, Greater Than 25% Ownership
Schedule of Available-for-sale Securities [Line Items]
Investment in unconsolidated subsidiaries	31,300		30,800
Other Limited Partnerships and Limited Liability Company Less than 25% Ownership
Schedule of Available-for-sale Securities [Line Items]
Investment in unconsolidated subsidiaries	$ 283,500		$ 209,100
Maximum | Tax Credit Partnerships Almost 100% Ownership
Schedule of Available-for-sale Securities [Line Items]
Investment ownership percentage	100.00%
Maximum | Tax Credit Partnerships Less Than 20% Ownership
Schedule of Available-for-sale Securities [Line Items]
Investment ownership percentage	20.00%
Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Required FAL Deposit	$ 123,500
Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Required FAL Deposit	$ 400
Non Government-Backed
Schedule of Available-for-sale Securities [Line Items]
Number of debt securities in unrealized loss position | Security	943		629
Debt securities in unrealized loss position as percentage of total debt securities held	44.90%		26.50%
Number of issuers in unrealized loss position | Issuer	494		375
Single greatest unrealized loss position	$ 500		$ 400
Second greatest unrealized loss position	400		$ 300
Revolving Credit Agreement
Schedule of Available-for-sale Securities [Line Items]
Securities pledged as collateral	$ 120,100