Postretirement Benefit Plans - Schedule of Assets at Fair Value for Defined-Benefit Pension Plans (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 330,404	$ 307,596
Percent of total pension assets	100.00%	100.00%
Other fixed maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 276	$ 270
Percent of total pension assets	0.00%	0.00%
Equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 127,708	$ 123,428
Percent of total pension assets	39.00%	40.00%
Guaranteed annuity contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 17,426	$ 17,082
Percent of total pension assets	5.00%	6.00%
Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 8,287	$ 4,842
Percent of total pension assets	3.00%	2.00%
Corporate debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 171,896	$ 146,381
Percent of total pension assets	52.00%	47.00%
Short-term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 4,811	$ 15,593
Percent of total pension assets	1.00%	5.00%