Schedule I - Condensed Financial Information of Registrant (Condensed Statements of Comprehensive Income) (Details) - USD ($) $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2015	[1],[2]	Sep. 30, 2015	[3],[4]	Jun. 30, 2015	[5],[6]	Mar. 31, 2015	[7],[8]	Dec. 31, 2014	[9],[10]	Sep. 30, 2014	[11],[12]	Jun. 30, 2014	[13],[14]	Mar. 31, 2014	[15]	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Net income	$ 37,100		$ 60,300		$ 58,400		$ 30,000		$ 4,900		$ (8,900)		$ 120,900		$ 28,800		$ 185,677	$ 145,629	$ 170,126
Unrealized gains (losses) on investments available-for-sale:
Unrealized holding (losses) gains arising during the period																	(10,480)	59,073	(78,217)
Less: Reclassification adjustments for gains included in earnings																	(192)	(2,710)	(23,975)
Unrealized (losses) gains on investments available-for-sale, net																	(10,672)	56,363	(102,192)
Defined benefit pension plans:
Prior service cost arising during the period																	0	0	607
Net gain (loss) arising during the period																	6,416	(11,893)	7,294
Less: Amortization of prior service cost and net loss included in net periodic pension cost																	2,560	600	2,572
Defined benefit pension plans, net																	(8,976)	11,293	(10,473)
Other comprehensive (loss) income, before tax																	(1,696)	45,070	(91,719)
Income tax (benefit) expense related to components of other comprehensive income																	(306)	15,464	(31,868)
Other comprehensive (loss) income, net of tax																	(1,390)	29,606	(59,851)
Comprehensive income																	184,287	175,235	110,275
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net income																	185,677	145,629	170,126
Unrealized gains (losses) on investments available-for-sale:
Unrealized holding (losses) gains arising during the period																	(10,480)	59,073	(78,217)
Less: Reclassification adjustments for gains included in earnings																	(192)	(2,710)	(23,975)
Unrealized (losses) gains on investments available-for-sale, net																	(10,672)	56,363	(102,192)
Defined benefit pension plans:
Prior service cost arising during the period																	0	0	607
Net gain (loss) arising during the period																	6,416	(11,893)	7,294
Less: Amortization of prior service cost and net loss included in net periodic pension cost																	2,560	600	2,572
Defined benefit pension plans, net																	(8,976)	11,293	(10,473)
Other comprehensive (loss) income, before tax																	(1,696)	45,070	(91,719)
Income tax (benefit) expense related to components of other comprehensive income																	(306)	15,464	(31,868)
Other comprehensive (loss) income, net of tax																	(1,390)	29,606	(59,851)
Comprehensive income																	$ 184,287	$ 175,235	$ 110,275

[1] Includes $22.7 million in pretax expenses primarily related to the Cognizant Transaction.

[2] Includes $58.2 million amortization of deferred costs of health insurer's fee and $21.1 million in other ACA fees.

[3] Includes $21.7 million in pretax expenses primarily related to the Cognizant Transaction.

[4] Includes $58.4 million amortization of deferred costs of health insurer's fee and $22.2 million in other ACA fees.

[5] Includes $26.4 million in pretax expenses primarily related to the Cognizant Transaction.

[6] Includes $58.5 million amortization of deferred costs of health insurer's fee and $21.5 million in other ACA fees.

[7] Includes $1.9 million pretax asset impairment and $44.3 million in pretax expenses primarily related to the Cognizant Transaction.

[8] Includes $57.8 million amortization of deferred costs of health insurer's fee and $20.8 million in other ACA fees.

[9] Includes $3.8 million pretax asset impairment primarily related to our assets held for sale in connection with the Cognizant Transaction and $68.3 million in pretax expenses primarily related to the Cognizant Transaction.

[10] Includes $35.4 million amortization of deferred costs of health insurer's fee and $25.8 million in other ACA fees.

[11] Includes $31.9 million amortization of deferred costs of health insurer's fee and $26.6 million in other ACA fees.

[12] Includes $84.7 million pretax asset impairment related to our assets held for sale in connection with the Cognizant Transaction and $21.1 million in pretax expenses primarily related to the Cognizant transaction.

[13] Includes $37.8 million amortization of deferred costs of health insurer's fee and $22.5 million in other ACA fees.

[14] Includes tax benefit of $72.6 million, net of adjustments, as a result of a loss on the stock of one of our subsidiaries.

[15] Includes $36.3 million amortization of deferred costs of health insurer's fee and $22.5 million in other ACA fees.